dm: fold __clone_and_map_data_bio into __split_and_process_bio

Fold __clone_and_map_data_bio into its only caller.

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