There's a race between cachefiles_mark_object_inactive() and
cachefiles_cull():
(1) cachefiles_cull() can't delete a backing file until the cache object
is marked inactive, but as soon as that's the case it's fair game.
(2) cachefiles_mark_object_inactive() marks the object as being inactive
and *only then* reads the i_blocks on the backing inode - but
cachefiles_cull() might've managed to delete it by this point.
Fix this by making sure cachefiles_mark_object_inactive() gets any data it
needs from the backing inode before deactivating the object.