nd6_cache_lladdr ignores lladdrlen parameter, uses ifp->if_addrlen for bcopy

Summary

nd6_cache_lladdr(:1768-1769) bcopy(lladdr,LLADDR(sdl),ifp->if_addrlen) ignores lladdrlen param. Comment(:1700) defers validation to callers. All 5 current callers validate. Future caller forgetting -> over-read. API safety hardening.