skd: Enable request tags for the block layer queue

Use the request tag when allocating a skd_fitmsg_context or
skd_request_context such that the lists used to track free elements
can be eliminated. Swap the skd_end_request() and skd_release_req()
calls to avoid triggering a use-after-free. Remove
skd_fitmsg_context.state and .outstanding because FIT messages are
shared among requests and because updating a FIT message after a
request has finished whould trigger a use-after-free.
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>