Cleanup nd6_cache_lladdr(). No functional changes.

• Since new extries are now allocated explicitly, fill in all the necessary fields for lle _before_ attaching it to the table.
• Remove ND6_LLINFO_INCOMPLETE check which was unused even in first KAME merge (r53541).
• After that, the only new state that function can set, was ND6_LLINFO_STALE. Given everything above, simplify logic besides do_update and is_newentry.
• Fix nd_resolve() comment.