NVMe: Allocate queues only for online cpus

The driver previously requested allocating queues for the total possible
number of CPUs so that blk-mq could rebalance these if CPUs were added
after initialization. The number of hardware contexts can now be changed
at runtime, so we only need to allocate the number of online queues
since we can add more later.

Suggested-by: Jeff Lien <jeff.lien@hgst.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@fb.com>