selftests: ublk: set queue pthread's cpu affinity

In NUMA machine, ublk IO performance is very sensitive with queue
pthread's affinity setting.

Retrieve queue's affinity and select the 1st cpu as queue thread's sched
affinity, and it is observed that single cpu task affinity can get
stable & good performance if client application is put on proper cpu.

Dump this info when adding one ublk device. Use shmem to communicate
queue's tid between parent and daemon.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-9-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>