dm: fix comment in dm_process_bio()

Refer to the correct function (->submit_bio instead of ->queue_bio).
Also, add details about why using blk_queue_split() isn't needed for
dm_wq_work()'s call to dm_process_bio().

Fixes: c62b37d9 ("block: move ->make_request_fn to struct block_device_operations")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>