nvme: refactor nvme_queue_rq

nvme: refactor nvme_queue_rq

This "backports" the structure I've used for the fabrics driver.  It
mostly started out as a cleanup so that I could actually understand
the code, but I think it also qualifies as a micro-optimization due
to the reduced time we hold q_lock and disable interrupts.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Orabug: 24691685
mainline commit ba1ca37ea4e320c108c356eb8c91ac652afc57dd
Conflicts:
    Adding GFP_ATOMIC to nvme_setup_prps and replacing
REQ_TYPE_DRV_PRIV with REQ_TYPE_SPECIAL

Signed-off-by:  Ashok Vairavan <ashok.vairavan@oracle.com>