netfilter: nfnetlink_hook: Use same family as request message

Use the same family as the request message, for consistency. The
netlink payload provides sufficient information to describe the hook
object, including the family.

This makes it easier to userspace to correlate the hooks are that
visited by the packets for a certain family.

Fixes: e2cf17d3

 ("netfilter: add new hook nfnl subsystem")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>