dm: fix redundant IO accounting for bios that need splitting

dm: fix redundant IO accounting for bios that need splitting

The risk of redundant IO accounting was not taken into consideration
when commit 18a25da84354 ("dm: ensure bio submission follows a
depth-first tree walk") introduced IO splitting in terms of recursion
via generic_make_request().

Fix this by subtracting the split bio's payload from the IO stats that
were already accounted for by start_io_acct() upon dm_make_request()
entry.  This repeat oscillation of the IO accounting, up then down,
isn't ideal but refactoring DM core's IO splitting to pre-split bios
_before_ they are accounted turned out to be an excessive amount of
change that will need a full development cycle to refine and verify.

Before this fix:

  /dev/mapper/stripe_dev is a 4-way stripe using a 32k chunksize, so
  bios are split on 32k boundaries.

  # fio --name=16M --filename=/dev/mapper/stripe_dev --rw=write --bs=64k --size=16M \
    	--iodepth=1 --ioengine=libaio --direct=1 --refill_buffers

  with debugging added:
  [103898.310264] device-mapper: core: start_io_acct: dm-2 WRITE bio->bi_iter.bi_sector=0 len=128
  [103898.318704] device-mapper: core: __split_and_process_bio: recursing for following split bio:
  [103898.329136] device-mapper: core: start_io_acct: dm-2 WRITE bio->bi_iter.bi_sector=64 len=64
  ...

  16M written yet 136M (278528 * 512b) accounted:
  # cat /sys/block/dm-2/stat | awk '{ print $7 }'
  278528

After this fix:

  16M written and 16M (32768 * 512b) accounted:
  # cat /sys/block/dm-2/stat | awk '{ print $7 }'
  32768

Fixes: 18a25da84354 ("dm: ensure bio submission follows a depth-first tree walk")
Cc: stable@vger.kernel.org # 4.16+
Reported-by: Bryan Gurney <bgurney@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>