Wire user buffers before acquiring the prefix list rwlock.
- We must rlock the prefix list and wlock the router list (in that
  order) before calling defrouterlist_del() in order to prevent LORs,
  since deleting a prefix involves acquiring both locks.
- Only acquire a reader lock on the prefix list if we don't already have
  a writer lock in pfxlist_onlink_check() since this function is called
  from prelist_remove().
Remove an unnecessary variable initialization I added earlier.