dm: rename split functions

Rename __split_and_process_bio to dm_split_and_process_bio.
Rename __split_and_process_non_flush to __split_and_process_bio.

Also fix a stale comment and whitespace.

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