skd: Introduce skd_process_request()

The only functional change in this patch is that the skd_fitmsg_context
in which requests are accumulated is changed from a local variable into
a member of struct skd_device. This patch will make the blk-mq conversion
easier.

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>