netfilter: ipv4: fix NULL dereference

Commit fa50d974 ("ipv4: Namespaceify ip_default_ttl sysctl knob")
use sock_net(skb->sk) to get the net namespace, but we can't assume
that sk_buff->sk is always exist, so when it is NULL, oops will happen.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Reviewed-by: Nikolay Borisov <kernel@kyup.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>