Unify nd6 state switching by using newly-created nd6_llinfo_setstate()
function. The change is mostly mechanical with the following exception:
Last piece of nd6_resolve_slow() was refactored: ND6_LLINFO_PERMANENT
condition was removed as always-true, explicit ND6_LLINFO_NOSTATE -> ND6_LLINFO_INCOMPLETE state transition was removed as duplicate.
Reviewed by: ae
Sponsored by: Yandex LLC