selftests: ublk: add stress test for per io daemons

selftests: ublk: add stress test for per io daemons

Add a new test_stress_06 for the per io daemons feature. This is just a
copy of test_stress_01 with the per_io_tasks flag added, with varying
amounts of nthreads. This test is able to reproduce a panic which was
caught manually during development [1]; in the current version of this
patch set, it passes.

Note that this commit also makes all stress tests using the
run_io_and_remove helper more stressful by additionally exercising the
batch submit (queue_rqs) path.

[1] https://lore.kernel.org/linux-block/aDgwGoGCEpwd1mFY@fedora/

Suggested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Link: https://lore.kernel.org/r/20250529-ublk_task_per_io-v8-8-e9d3b119336a@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>