* Move lle creation/deletion from lla_lookup to separate functions:
  lla_lookup(LLE_CREATE) -> lla_create
  lla_lookup(LLE_DELETE) -> lla_delete
  Assume lla_create to return LLE_EXCLUSIVE lock for lle.
* Rework lla_rt_output to perform all lle changes under afdata WLOCK.
* change arp_ifscrub() ackquire afdata WLOCK, the same as arp_ifinit().