dm: refactor dm_split_and_process_bio a bit

Remove needless branching and indentation. Leaves code to catch
malformed op_is_zone_mgmt bios (they shouldn't have a payload).

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>