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: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NKeith Busch <keith.busch@intel.com>
Signed-off-by: NJens Axboe <axboe@fb.com>