cachefiles: Fix race between inactivating and culling a cache object

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.

Without this, the following oops may occur:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000098
IP: [<ffffffffa06c5cc1>] cachefiles_mark_object_inactive+0x61/0xb0 [cachefiles]
...
CPU: 11 PID: 527 Comm: kworker/u64:4 Tainted: G          I    ------------   3.10.0-470.el7.x86_64 #1
Hardware name: Hewlett-Packard HP Z600 Workstation/0B54h, BIOS 786G4 v0...