ubifs: have ubifs_update_time use inode_update_timestamps

In later patches, we're going to drop the "now" parameter from the
update_time operation. Prepare ubifs for this, by having it use the new
inode_update_timestamps helper.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230807-mgctime-v7-6-d1dec143a704@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>