md/raid10: fix the condition to call bio_end_io_acct()

/sys/block/[device]/queue/iostats is used to control whether to count io
stat. Write 0 to it will clear queue_flags QUEUE_FLAG_IO_STAT which means
iostats is disabled. If we disable iostats and later endable it, the io
issued during this period will be counted incorrectly, inflight will be
decreased to -1.

//T1 set iostats
echo 0 > /sys/block/md0/queue/iostats
clear QUEUE_FLAG_IO_STAT

//T2 issue io
if (QUEUE_FLAG_IO_STAT) -> false
bio_start_io_acct
inflight++

echo 1 > /sys/block/md0/queue/iostats
set QUEUE_FLAG_IO_STAT

//T3 io end
if (QUEUE_FLAG_IO_STAT) -> true
bio_end_io_acct
inflight-- -> -1

Also, if iostats is enabled while issuing io but disabled while io end,
inflight will never be decreased.

Fix it by checking start_time when io end. If start_time is not 0, call
bio_end_io_acct().

Fixes: 528bc2cf2fcc ("md/raid10: enable io accounting")
Signed-off-by: Li Nan <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/md/raid10.c

···
 	if (!test_bit(R10BIO_Uptodate, &r10_bio->state))
 		bio->bi_status = BLK_STS_IOERR;
 
-	if (blk_queue_io_stat(bio->bi_bdev->bd_disk->queue))
+	if (r10_bio->start_time)
 		bio_end_io_acct(bio, r10_bio->start_time);
 	bio_endio(bio);
 	/*