netfilter: nat: Avoid ABI change in 4.13.13

netfilter: nat: Avoid ABI change in 4.13.13

Commit e1bf1687740c ("netfilter: nat: Revert "netfilter: nat: convert
nat bysrc hash to rhashtable") changed the type of
nf_conn::nat_bysource.  Thankfully the two types are the same size,
and nothing outside of nf_nat_core touches this field directly.  Hide
the change from genksyms but add compile-time assertions to make sure
that this isn't an ABI change.

Gbp-Pq: Topic debian
Gbp-Pq: Name netfilter-nat-avoid-abi-change-in-4.13.13.patch