fs/writeback: only calculate dirtied_before when b_io is empty

The dirtied_before is only used when b_io is not empty, so only calculate
when b_io is not empty.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20240228091958.288260-5-shikemeng@huaweicloud.com


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>