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>

drivers/md/raid10.c

@@ -1735,6 +1735,7 @@ 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;