nvme/pci Limit write queue sizes to possible cpus

The driver can never use more queues of any type than the number of
possible CPUs, so a higher value causes the driver to allocate more
memory for IO queues than it could ever use. Limit the parameter at
module load time to the number of possible cpus.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <kbusch@kernel.org>