ipv6: tcp: fix potential NULL deref in tcp_v6_send_reset()

syzbot found a crash in tcp_v6_send_reset() caused by my latest
change.

Problem is that if an skb has been queued to socket prequeue,
skb_dst(skb)->dev can not anymore point to the device.

Fortunately in this case the socket pointer is not NULL.

A similar issue has been fixed in commit 0f85feae ("tcp: fix
more NULL deref after prequeue changes"), I should have known better.

Fixes: 323a53c4

 ("ipv6: tcp: enable flowlabel reflection in some RST packets")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>