dm integrity: use plugging when writing the journal

When copying data from the journal to the appropriate place, we submit
many IOs.  Some of these IOs could go to adjacent areas.  Use on-stack
plugging so that adjacent IOs get merged during submission.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>