netfilter: use NFPROTO_IPV4 instead of AF_INET

The field family of xt_target should be NFPROTO_IPV4, though
NFPROTO_IPV4 and AF_INET are the same.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>