ip6mr: Fix skb_under_panic in ip6mr_cache_report()

 skbuff: skb_under_panic: text:ffffffff88771f69 len:56 put:-4
 head:ffff88805f86a800 data:ffff887f5f86a850 tail:0x88 end:0x2c0 dev:pim6reg
 ------------[ cut here ]------------
 kernel BUG at net/core/skbuff.c:192!
 invalid opcode: 0000 [#1] PREEMPT SMP KASAN
 CPU: 2 PID: 22968 Comm: kworker/2:11 Not tainted 6.5.0-rc3-00044-g0a8db05b571a torvalds#236
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
 Workqueue: ipv6_addrconf addrconf_dad_work
 RIP: 0010:skb_panic+0x152/0x1d0
 Call Trace:
  <TASK>
  skb_push+0xc4/0xe0
  ip6mr_cache_report+0xd69/0x19b0
  reg_vif_xmit+0x406/0x690
  dev_hard_start_xmit+0x17e/0x6e0
  __dev_queue_xmit+0x2d6a/0x3d20
  vlan_dev_hard_start_xmit+0x3ab/0x5c0
  dev_hard_start_xmit+0x17e/0x6e0
  __dev_queue_xmit+0x2d6a/0x3d20
  neigh_connected_output+0x3ed/0x570
  ip6_finish_output2+0x5b5/0x1950
  ip6_finish_output+0x693/0x11c0
  ip6_output+0x24b/0x880
  NF_HOOK.constprop.0+0xfd/0x530
  ndisc_send_skb+0x9db/0x1400
  ndisc_send_rs+0x12a/0x6c0
  addrconf_dad_completed+0x3c9/0xea0
  addrconf_dad_work+0x849/0x1420
  process_one_work+0xa22/0x16e0
  worker_thread+0x679/0x10c0
  ret_from_fork+0x28/0x60
  ret_from_fork_asm+0x11/0x20

When setup a vlan device on dev pim6reg, DAD ns packet may sent on reg_vif_xmit().
reg_vif_xmit()
    ip6mr_cache_report()
        skb_push(skb, -skb_network_offset(pkt));//skb_network_offset(pkt) is 4
And skb_push declar as this:
	void *skb_push(struct sk_buff *skb, unsigned int len);
		skb->data -= len;
		//0xffff888f5f86a84c - 0xfffffffc = 0xffff887f5f86a850
skb->data is set to 0xffff887f5f86a850, which is invalid mem addr, lead to skb_push() fails.

Fixes: 14fb64e ("[IPV6] MROUTE: Support PIM-SM (SSM).")
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>

net/ipv6/ip6mr.c

@@ -1051,9 +1051,9 @@ static int ip6mr_cache_report(const struct mr_table *mrt, struct sk_buff *pkt,
 	int ret;
 
 #ifdef CONFIG_IPV6_PIMSM_V2
+	int nhoff = skb_network_offset(pkt);
 	if (assert == MRT6MSG_WHOLEPKT || assert == MRT6MSG_WRMIFWHOLE)
-		skb = skb_realloc_headroom(pkt, -skb_network_offset(pkt)
-						+sizeof(*msg));
+		skb = skb_realloc_headroom(pkt, -nhoff + sizeof(*msg));
 	else
 #endif
 		skb = alloc_skb(sizeof(struct ipv6hdr) + sizeof(*msg), GFP_ATOMIC);
@@ -1073,7 +1073,8 @@ static int ip6mr_cache_report(const struct mr_table *mrt, struct sk_buff *pkt,
 		   And all this only to mangle msg->im6_msgtype and
 		   to set msg->im6_mbz to "mbz" :-)
 		 */
-		skb_push(skb, -skb_network_offset(pkt));
+		skb->data += nhoff;
+		skb->len  -= nhoff;
 
 		skb_push(skb, sizeof(*msg));
 		skb_reset_transport_header(skb);