Some fallout from the pagemap_lru_lock changes:

- lru_cache_del() is no longer used.  Kill it.

- page_cache_release() almost never actually frees pages.  So inline
  page_cache_release() and move its rarely-called slow path into (the
  misnamed) mm/swap.c

- update the locking comment in filemap.c.  pagemap_lru_lock used to
  be one of the outermost locks in the VM locking hierarchy.  Now, we
  never take any other locks while holding pagemap_lru_lock.  So it
  doesn't have any relationship with anything.

- put_page() now removes pages from the LRU on the final put.  The
  lock is interrupt safe.