all protocols did set this to nf_nat_l4proto_nlattr_to_range, so
just call it directly.

The important difference is that we'll now also call it for
protocols that we don't support (i.e., nf_nat_proto_unknown did
not provide .nlattr_to_range).

However, there should be no harm, even icmp provided this callback.
If we don't implement a specific l4nat for this, nothing would make
use of this information, so adding a big switch/case construct listing
all supported l4protocols seems a bit pointless.

This change leaves a single function pointer in the l4proto struct.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>