CacheFiles: Implement interface to check cache consistency

Implement the FS-Cache interface to check the consistency of a cache object in
CacheFiles.

Original-author: Hongyi Jia <jiayisuse@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Hongyi Jia <jiayisuse@gmail.com>
cc: Milosz Tanski <milosz@adfin.com>

diff --cc fs/cachefiles/interface.c
Simple merge

diff --cc fs/cachefiles/internal.h
Simple merge

diff --cc fs/cachefiles/xattr.c
Simple merge