nvme: merge iod and cmd_info

BugLink: http://bugs.launchpad.net/bugs/1531539

Merge the two per-request structures in the nvme driver into a single
one.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit f4800d6d)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>