FS-Cache: Add interface to check consistency of a cached object

Extend the fscache netfs API so that the netfs can ask as to whether a cache
object is up to date with respect to its corresponding netfs object:

	int fscache_check_consistency(struct fscache_cookie *cookie)

This will call back to the netfs to check whether the auxiliary data associated
with a cookie is correct.  It returns 0 if it is and -ESTALE if it isn't; it
may also return -ENOMEM and -ERESTARTSYS.

The backends now have to implement a mandatory operation pointer:

	int (*check_consistency)(struct fscache_object *object)

that corresponds to the above API call.  FS-Cache takes care of pinning the
object and the cookie in memory and managing this call with respect to the
object state.

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>

include/linux/fscache-cache.h

@@ -251,6 +251,10 @@ struct fscache_cache_ops {
 	/* unpin an object in the cache */
 	void (*unpin_object)(struct fscache_object *object);
 
+	/* check the consistency between the backing cache and the FS-Cache
+	 * cookie */
+	bool (*check_consistency)(struct fscache_operation *op);
+
 	/* store the updated auxiliary data on an object */
 	void (*update_object)(struct fscache_object *object);