fs/writeback: remove unnecessary return in writeback_inodes_sb

writeback_inodes_sb doesn't have return value, just remove unnecessary
return in it.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20240228091958.288260-7-shikemeng@huaweicloud.com


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