md/raid10: fix missing discard IO accounting

JIRA: https://issues.redhat.com/browse/RHEL-106845

commit d05af90
Author: Yu Kuai <yukuai3@huawei.com>
Date:   Tue Mar 25 09:57:46 2025 +0800

    md/raid10: fix missing discard IO accounting

    md_account_bio() is not called from raid10_handle_discard(), now that we
    handle bitmap inside md_account_bio(), also fix missing
    bitmap_startwrite for discard.

    Test whole disk discard for 20G raid10:

    Before:
    Device   d/s     dMB/s   drqm/s  %drqm d_await dareq-sz
    md0    48.00     16.00     0.00   0.00    5.42   341.33

    After:
    Device   d/s     dMB/s   drqm/s  %drqm d_await dareq-sz
    md0    68.00  20462.00     0.00   0.00    2.65 308133.65

    Link: https://lore.kernel.org/linux-raid/20250325015746.3195035-1-yukuai1@huaweicloud.com
    Fixes: 528bc2c ("md/raid10: enable io accounting")
    Signed-off-by: Yu Kuai <yukuai3@huawei.com>
    Acked-by: Coly Li <colyli@kernel.org>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

drivers/md/raid10.c

@@ -1744,6 +1744,7 @@ static int raid10_handle_discard(struct mddev *mddev, struct bio *bio)
 	 * The discard bio returns only first r10bio finishes
 	 */
 	if (first_copy) {
+		md_account_bio(mddev, &bio);
 		r10_bio->master_bio = bio;
 		set_bit(R10BIO_Discard, &r10_bio->state);
 		first_copy = false;