null_blk: set use_per_node_hctx param to false

The defaults for the module is to instantiate itself with blk-mq and a
submit queue for each CPU node in the system.

To save resources, initialize instead with a single submit queue.

Signed-off-by: Matias Bjorling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>