SUNRPC: Fix null-ptr-deref when xps sysfs alloc failed

There is a null-ptr-deref when xps sysfs alloc failed:
  BUG: KASAN: null-ptr-deref in sysfs_do_create_link_sd+0x40/0xd0
  Read of size 8 at addr 0000000000000030 by task gssproxy/457

  CPU: 5 PID: 457 Comm: gssproxy Not tainted 6.0.0-09040-g02357b27ee03 #9
  Call Trace:
   <TASK>
   dump_stack_lvl+0x34/0x44
   kasan_report+0xa3/0x120
   sysfs_do_create_link_sd+0x40/0xd0
   rpc_sysfs_client_setup+0x161/0x1b0
   rpc_new_client+0x3fc/0x6e0
   rpc_create_xprt+0x71/0x220
   rpc_create+0x1d4/0x350
   gssp_rpc_create+0xc3/0x160
   set_gssp_clnt+0xbc/0x140
   write_gssp+0x116/0x1a0
   proc_reg_write+0xd6/0x130
   vfs_write+0x177/0x690
   ksys_write+0xb9/0x150
   do_syscall_64+0x35/0x80
   entry_SYSCALL_64_after_hwframe+0x46/0xb0

When the xprt_switch sysfs alloc failed, should not add xprt and
switch sysfs to it, otherwise, maybe null-ptr-deref; also initialize
the 'xps_sysfs' to NULL to avoid oops when destroy it.

Fixes: 2a338a5 ("sunrpc: add a symlink from rpc-client directory to the xprt_switch")
Fixes: d408ebe ("sunrpc: add add sysfs directory per xprt under each xprt_switch")
Fixes: baea994 ("sunrpc: add xprt_switch direcotry to sunrpc's sysfs")
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/sysfs.c

@@ -518,13 +518,16 @@ void rpc_sysfs_client_setup(struct rpc_clnt *clnt,
 			    struct net *net)
 {
 	struct rpc_sysfs_client *rpc_client;
+	struct rpc_sysfs_xprt_switch *xswitch =
+		(struct rpc_sysfs_xprt_switch *)xprt_switch->xps_sysfs;
+
+	if (!xswitch)
+		return;
 
 	rpc_client = rpc_sysfs_client_alloc(rpc_sunrpc_client_kobj,
 					    net, clnt->cl_clid);
 	if (rpc_client) {
 		char name[] = "switch";
-		struct rpc_sysfs_xprt_switch *xswitch =
-			(struct rpc_sysfs_xprt_switch *)xprt_switch->xps_sysfs;
 		int ret;
 
 		clnt->cl_sysfs = rpc_client;
@@ -558,6 +561,8 @@ void rpc_sysfs_xprt_switch_setup(struct rpc_xprt_switch *xprt_switch,
 		rpc_xprt_switch->xprt_switch = xprt_switch;
 		rpc_xprt_switch->xprt = xprt;
 		kobject_uevent(&rpc_xprt_switch->kobject, KOBJ_ADD);
+	} else {
+		xprt_switch->xps_sysfs = NULL;
 	}
 }
 
@@ -569,6 +574,9 @@ void rpc_sysfs_xprt_setup(struct rpc_xprt_switch *xprt_switch,
 	struct rpc_sysfs_xprt_switch *switch_obj =
 		(struct rpc_sysfs_xprt_switch *)xprt_switch->xps_sysfs;
 
+	if (!switch_obj)
+		return;
+
 	rpc_xprt = rpc_sysfs_xprt_alloc(&switch_obj->kobject, xprt, gfp_flags);
 	if (rpc_xprt) {
 		xprt->xprt_sysfs = rpc_xprt;