scsi: use block layer helpers to calculate num of queues

scsi: use block layer helpers to calculate num of queues

The calculation of the upper limit for queues does not depend solely on
the number of online CPUs; for example, the isolcpus kernel
command-line option must also be considered.

To account for this, the block layer provides a helper function to
retrieve the maximum number of queues. Use it to set an appropriate
upper queue number limit.

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Daniel Wagner <wagi@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20250617-isolcpus-queue-counters-v1-4-13923686b54b@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>