skd: more removal of bio-based code

Remove skd_flush_cmd structure and skd_flush_slab.
Remove skd_end_request wrapper around skd_end_request_blk.
Remove skd_requeue_request, use blk_requeue_request directly.
Cleanup some comments (remove "bio" info) and whitespace.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>