skd: Coalesce struct request and struct skd_request_context

Set request_queue.cmd_size, introduce skd_init_rq() and skd_exit_rq()
and remove skd_device.skreq_table.

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>