nvme: special case AEN requests

AEN requests are different from other requests in that they don't time out
or can easily be cancelled.  Because of that we should not use the blk-mq
infrastructure but just special case them in the completion path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>