cachefiles: fix incorrect dentry refcount in cachefiles_cull()

In the Linux kernel, the following vulnerability has been resolved:

cachefiles: fix incorrect dentry refcount in cachefiles_cull()

The patch mentioned below changed cachefiles_bury_object() to expect 2

references to the 'rep' dentry. Three of the callers were changed to

use start_removing_dentry() which takes an extra reference so in those

cases the call gets the expected references.

However there is another call to cachefiles_bury_object() in

cachefiles_cull() which did not need to be changed to use

start_removing_dentry() and so was not properly considered.

It still passed the dentry with just one reference so the net result is

that a reference is lost.

To meet the expectations of cachefiles_bury_object(), cachefiles_cull()

must take an extra reference before the call. It will be dropped by

cachefiles_bury_object().