dm: backfill abnormal IO support to non-splitting IO submission

Otherwise, these abnormal IOs would be sent to the DM target
regardless of whether the target advertised support for them.

Factor out __process_abnormal_io() from __split_and_process_non_flush()
so that discards, write same, etc may be conditionally processed.

Fixes: 978e51ba ("dm: optimize bio-based NVMe IO submission")
Cc: stable@vger.kernel.org # 4.16
Signed-off-by: Mike Snitzer <snitzer@redhat.com>