netfilter: nat: remove l4proto->in_range

With exception of icmp, all of the l4 nat protocols set this to
nf_nat_l4proto_in_range.

Get rid of this and just check the l4proto in the caller.

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