CacheFiles: Implement invalidation

Implement invalidation for CacheFiles.  This is in two parts:

 (1) Provide an invalidation method (which just truncates the backing file).

 (2) Abort attempts to copy anything read from the backing file whilst
     invalidation is in progress.

Question: CacheFiles uses truncation in a couple of places.  It has been using
notify_change() rather than sys_truncate() or something similar.  This means
it bypasses a bunch of checks and suchlike that it possibly should be making
(security, file locking, lease breaking, vfsmount write).  Should it be using
vfs_truncate() as added by a preceding patch or should it use notify_write()
and assume that anyone poking around in the cache files on disk gets
everything they deserve?

Signed-off-by: David Howells <dhowells@redhat.com>

fs/cachefiles/rdwr.c

@@ -174,7 +174,10 @@ static void cachefiles_read_copier(struct fscache_operation *_op)
 		_debug("- copy {%lu}", monitor->back_page->index);
 
 	recheck:
-		if (PageUptodate(monitor->back_page)) {
+		if (test_bit(FSCACHE_COOKIE_INVALIDATING,
+			     &object->fscache.cookie->flags)) {
+			error = -ESTALE;
+		} else if (PageUptodate(monitor->back_page)) {
 			copy_highpage(monitor->netfs_page, monitor->back_page);
 			fscache_mark_page_cached(monitor->op,
 						 monitor->netfs_page);