netfilter: nft_nat: correct length for loading protocol registers

netfilter: nft_nat: correct length for loading protocol registers

The values in the protocol registers are two bytes wide.  However, when
parsing the register loads, the code currently uses the larger 16-byte
size of a `union nf_inet_addr`.  Change it to use the (correct) size of
a `union nf_conntrack_man_proto` instead.

Fixes: d07db9884a5f ("netfilter: nf_tables: introduce nft_validate_register_load()")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>