null_blk: allow zero poll queues

There isn't any reason to not allow zero poll queues from user
viewpoint.

Also sometimes we need to compare io poll between poll mode and irq
mode, so not allowing poll queues is bad.

Fixes: 15dfc662 ("null_blk: Fix handling of submit_queues and poll_queues attributes")
Cc: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20211203023935.3424042-1-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>