ipv6: make icmp6_send() robust against null skb->dev

ipv6: make icmp6_send() robust against null skb->dev

syzbot was able to crash one host with the following stack trace :

kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 8625 Comm: syz-executor4 Not tainted 4.20.0+ #8
RIP: 0010:dev_net include/linux/netdevice.h:2169 [inline]
RIP: 0010:icmp6_send+0x116/0x2d30 net/ipv6/icmp.c:426
 icmpv6_send
 smack_socket_sock_rcv_skb
 security_sock_rcv_skb
 sk_filter_trim_cap
 __sk_receive_skb
 dccp_v6_do_rcv
 release_sock

This is because a RX packet found socket owned by user and
was stored into socket backlog. Before leaving RCU protected section,
skb->dev was cleared in __sk_receive_skb(). When socket backlog
was finally handled at release_sock() time, skb was fed to
smack_socket_sock_rcv_skb() then icmp6_send()

We could fix the bug in smack_socket_sock_rcv_skb(), or simply
make icmp6_send() more robust against such possibility.

In the future we might provide to icmp6_send() the net pointer
instead of infering it.

Fixes: d9d18de8d0fa ("Smack: Inform peer that IPv6 traffic has been blocked")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Piotr Sawicki <p.sawicki2@partner.samsung.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: David S. Miller <davem@davemloft.net>