netfilter: nf_tables: reject nat hook registration if prio is before conntrack

No problem for iptables as priorities are fixed values defined in the
nat modules, but in nftables the priority its coming from userspace.

Reject in case we see that such a hook would not work.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>