NVMe: Use unbounded work queue for all work

Removes all usage of the global work queue so work can't be
scheduled on two different work queues, and removes nvme's work queue
singlethreadedness so controllers can be driven in parallel.

Signed-off-by: Keith Busch <keith.busch@intel.com>
[hch: keep the dead controller removal on the system workqueue to avoid
 deadlocks]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>