netfilter: ipv6: nf_defrag: drop skb dst before queueing

Eric Dumazet reports:
 Here is a reproducer of an annoying bug detected by syzkaller on our production kernel
 [..]
 ./b78305423 enable_conntrack
 Then :
 sleep 60
 dmesg | tail -10
 [  171.599093] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  181.631024] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  191.687076] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  201.703037] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  211.711072] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  221.959070] unregister_netdevice: waiting for lo to become free. Usage count = 2

Reproducer sends ipv6 fragment that hits nfct defrag via LOCAL_OUT hook.
skb gets queued until frag timer expiry -- 1 minute.

Normally nf_conntrack_reasm gets called during prerouting, so skb has
no dst yet which might explain why this wasn't spotted earlier.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>