netinet6: use IF_ADDR_LOCK instead of IF_AFDATA_LOCK in defrtr_ipv6_only_ifp

It is not clear what exactly this function is locking against. Seems
like just use some generic interface lock. The IF_AFDATA_LOCK goes
away soon together with if_afdata[], so put at least something in its
place. Note that this code is dead anyway (#ifdef EXPERIMENTAL).