Implimenting inet6_hash_frag because it has been static since Linux 3.13.
Its implementation is a copy of net/ipv6/reassembly.c's function of the same name.
Complement to issue87.
nf_hook.c was causing some compilation warnings in kernel 3.13 because the definition of nf_hookfn changed.
Missed this nugget in the previous commit.
Merge branch 'issue87'