dm: fix dm_wq_work() to only use __split_and_process_bio() if appropriate

Otherwise targets that don't support/expect IO splitting could resubmit
bios using code paths with unnecessary IO splitting complexity.

Depends-on: 24113d48 ("dm: avoid indirect call in __dm_make_request")
Fixes: 978e51ba

 ("dm: optimize bio-based NVMe IO submission")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>