netfilter: nf_nat: move alloc_null_binding to nf_nat_core.c

Similar to nat_decode_session, alloc_null_binding is needed for both
ip_tables and nf_tables, so move it to nf_nat_core.c. This change
is required by nf_tables.

This is an adapted version of the original patch from Patrick McHardy.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>