Fix condition for nd6_llinfo_getholdsrc() introduced in r287484.

Fix condition for nd6_llinfo_getholdsrc() introduced in r287484.
  Effectively it always returned NULL so SAS was always performed and
  sometimes the result might have been different.

Fix state machine change accidentally introduced in r287985:
  state (4) inside nd6_cache_lladdr() (existing entry got nd message
  with the same lladdress) started to cause lle state transition to STALE
  instead of no-action.