fscache: Fix error handling in fscache_begin_operation()

Fix fscache_begin_operation() to clear cres->cache_priv on error, otherwise
fscache_resources_valid() will report it as being valid.
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/3933237.1710514106@warthog.procyon.org.ukReviewed-by: Jeff Layton <jlayton@kernel.org>
Reported-by: Marc Dionne <marc.dionne@auristor.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/netfs/fscache_io.c

@@ -83,8 +83,10 @@ static int fscache_begin_operation(struct netfs_cache_resources *cres,
 	cres->debug_id		= cookie->debug_id;
 	cres->inval_counter	= cookie->inval_counter;
 
-	if (!fscache_begin_cookie_access(cookie, why))
+	if (!fscache_begin_cookie_access(cookie, why)) {
+		cres->cache_priv = NULL;
 		return -ENOBUFS;
+	}
 
 again:
 	spin_lock(&cookie->lock);