btrfs: filp_open() returns ERR_PTR() on failure, not NULL...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/btrfs/volumes.c

@@ -1485,7 +1485,7 @@ static void update_dev_time(char *path_name)
 	struct file *filp;
 
 	filp = filp_open(path_name, O_RDWR, 0);
-	if (!filp)
+	if (IS_ERR(filp))
 		return;
 	file_update_time(filp);
 	filp_close(filp, NULL);