btrfs: have it use inode_update_timestamps

In later patches, we're going to drop the "now" argument from the
update_time operation. Have btrfs_update_time use the new
inode_update_timestamps helper to fetch a new timestamp and update it
properly.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230807-mgctime-v7-4-d1dec143a704@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>