skd: Convert to blk-mq

Introduce a tag set and a blk_mq_ops structure. Set .cmd_size such
that struct request and struct skd_request_context are allocated
through a single allocation. Remove the skd_request_context.req
pointer. Make queue starting asynchronous such that this can occur
safely from interrupt context. Use locking to protect skdev->skmsg
and *skdev->skmsg against concurrent access from concurrent
.queue_rq() calls. Introduce the functions skd_init_request() and
skd_exit_request() to set up / clean up the per-request S/G-list.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>