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: 528bc2cf ("md/raid10: enable io accounting")
Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230609094320.2397604-1-linan666@huaweicloud.com