find_lock_entry — locate, pin and lock a page cache entry
| struct page * find_lock_entry ( | struct address_space * mapping, | 
| pgoff_t offset ); | 
   Looks up the page cache slot at mapping & offset.  If there is a
   page cache page, it is returned locked and with an increased
   refcount.
   
If the slot holds a shadow entry of a previously evicted page, or a swap entry from shmem/tmpfs, it is returned.
   Otherwise, NULL is returned.
   
   find_lock_entry may sleep.