Convert to IfAPI everything except `IF_AFDATA_WLOCK` usage in neigh.c.
I don't think it's a good idea to to expose locking as function-based API. I'd prefer to leave `neigh.c` with access to a private header for some time.
I two options here: 1) move actual LLE insertion code inside `if_lltable.c`
2) refactoring `IF_AFDATA` locking in general to have per-af lock instead of a global one.
Currently I'm leaning for the first option, which is fairly simple.