Fix problem in r238990 (MFC'd in r240313).  The LLE_LINKED flag should be
tested prior to entering llentry_free(), and in case if we lose the race,
we should simply perform LLE_FREE_LOCKED().  Otherwise, if the race is
lost by the thread performing arptimer(), it will remove two references
from the lle instead of one.

Reported by:	Ian FREISLICH <ianf clue.co.za>