Simplify if (lladdr) condition in nd6_cache_lladdr():

Simplify if (lladdr) condition in nd6_cache_lladdr():
  For case (7) (new entry) nothing has to be done except lle_event.
  Invoke this event directly from "create new lle" code block.
  For case (4) (existing entry, same mac) useless mac update was performed,
  along with LLENTRY_RESOLVED lle_event. There was no sense in doing that,
  since nothing really had changed. Simply avoid this condition instead.
  Given that, condition was simplified to (3),(5) states which can be merged
  with previous block.