NVMe: Correctly handle IOCTL_SUBMIT_IO when cpus > online queues

nvme_submit_io_cmd() uses smp_processor_id() to pick an IO queue index.
This patch fixes the case where there are more cpus from which the ioctl
call can originate than online queues, which can happen when a device
supports or was allocated fewer interrupt vectors than exist cpu cores.

Thanks to Keith Busch for the implementation suggestion.

Signed-off-by: Sam Bradshaw <sbradshaw@micron.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>