bgpd: Intern attributes before putting into rib-out

After we call subgroup_announce_check(), we leave communities, large-communities
that are modified by route-maps uninterned, and here we have a memory leak.

```
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323:Direct leak of 80 byte(s) in 2 object(s) allocated from:
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    #0 0x7f0858d90037 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:154
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    #1 0x7f08589b15b2 in qcalloc lib/memory.c:105
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#2 0x561f5c4e08d2 in lcommunity_new bgpd/bgp_lcommunity.c:28
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#3 0x561f5c4e11d9 in lcommunity_dup bgpd/bgp_lcommunity.c:141
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#4 0x561f5c5c3b8b in route_set_lcommunity bgpd/bgp_routemap.c:2491
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#5 0x7f0858a177a5 in route_map_apply_ext lib/routemap.c:2675
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#6 0x561f5c5696f9 in subgroup_announce_check bgpd/bgp_route.c:2352
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#7 0x561f5c5fb728 in subgroup_announce_table bgpd/bgp_updgrp_adv.c:682
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#8 0x561f5c5fbd95 in subgroup_announce_route bgpd/bgp_updgrp_adv.c:765
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#9 0x561f5c5f6105 in peer_af_announce_route bgpd/bgp_updgrp.c:2187
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#10 0x561f5c5790be in bgp_announce_route_timer_expired bgpd/bgp_route.c:5032
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#11 0x7f0858a76e4e in thread_call lib/thread.c:1991
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#12 0x7f0858974c24 in frr_run lib/libfrr.c:1185
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#13 0x561f5c3e955d in main bgpd/bgp_main.c:505
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#14 0x7f08583a9d09 in __libc_start_main ../csu/libc-start.c:308
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323:Indirect leak of 144 byte(s) in 2 object(s) allocated from:
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    #0 0x7f0858d8fe8f in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    #1 0x7f08589b1579 in qmalloc lib/memory.c:100
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#2 0x561f5c4e1282 in lcommunity_dup bgpd/bgp_lcommunity.c:144
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#3 0x561f5c5c3b8b in route_set_lcommunity bgpd/bgp_routemap.c:2491
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#4 0x7f0858a177a5 in route_map_apply_ext lib/routemap.c:2675
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#5 0x561f5c5696f9 in subgroup_announce_check bgpd/bgp_route.c:2352
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#6 0x561f5c5fb728 in subgroup_announce_table bgpd/bgp_updgrp_adv.c:682
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#7 0x561f5c5fbd95 in subgroup_announce_route bgpd/bgp_updgrp_adv.c:765
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#8 0x561f5c5f6105 in peer_af_announce_route bgpd/bgp_updgrp.c:2187
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#9 0x561f5c5790be in bgp_announce_route_timer_expired bgpd/bgp_route.c:5032
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#10 0x7f0858a76e4e in thread_call lib/thread.c:1991
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#11 0x7f0858974c24 in frr_run lib/libfrr.c:1185
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#12 0x561f5c3e955d in main bgpd/bgp_main.c:505
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-    FRRouting#13 0x7f08583a9d09 in __libc_start_main ../csu/libc-start.c:308
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-
./bgp_large_community.test_bgp_large_community_topo_2/r1.bgpd.asan.2465323-SUMMARY: AddressSanitizer: 224 byte(s) leaked in 4 allocation(s).
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

bgpd/bgp_updgrp_adv.c

@@ -683,11 +683,14 @@ void subgroup_announce_table(struct update_subgroup *subgrp,
 						    &attr, NULL)) {
 				/* Check if route can be advertised */
 				if (advertise) {
-					if (!bgp_check_withdrawal(bgp, dest))
+					if (!bgp_check_withdrawal(bgp, dest)) {
+						struct attr *adv_attr =
+							bgp_attr_intern(&attr);
+
 						bgp_adj_out_set_subgroup(
-							dest, subgrp, &attr,
+							dest, subgrp, adv_attr,
 							ri);
-					else
+					} else
 						bgp_adj_out_unset_subgroup(
 							dest, subgrp, 1,
 							bgp_addpath_id_for_peer(