nvme-pci: handle completions outside of the queue lock

nvme-pci: handle completions outside of the queue lock

Split the completion of events into a two part process:

1) Reap the events inside the queue lock
2) Complete the events outside the queue lock

Since we never wrap the queue, we can access it locklessly after we've
updated the completion queue head. This patch started off with batching
events on the stack, but with this trick we don't have to. Keith Busch
<keith.busch@intel.com> came up with that idea.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <keith.busch@intel.com>
[hch: refactored, restored poll early exit optimization]
Signed-off-by: Christoph Hellwig <hch@lst.de>