nvme: default to 0 poll queues

We need a better way of configuring this, and given that polling is
(still) a bit niche, let's default to using 0 poll queues. That way
we'll have the same read/write/poll behavior as 4.20, and users that
want to test/use polling are required to do manual configuration of the
number of poll queues.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>