selftests: ublk: make IO & device removal test more stressful

__run_io_and_remove() is used in several stress tests for running heavy
IO vs. removing device meantime.

However, sequential `readwrite` is taken in the fio script, which isn't
correct, we should take random IO for saturating ublk device.

Also turns out '--num_jobs=4' isn't stressful enough, so change it to
'--num_jobs=$(nproc)'.

Finally we don't cover single queue test in `test_stress_02.sh`, so add
single queue test which can trigger request tag recycling easier.

With above change the issue in #1 can be reproduced reliably in stress_02.sh.

Link:https://lore.kernel.org/linux-block/mruqwpf4tqenkbtgezv5oxwq7ngyq24jzeyqy4ixzvivatbbxv@4oh2wzz4e6qn/ #1

Cc: Jared Holzman <[e-mail]>
Cc: Shinichiro Kawasaki <[e-mail]>
Signed-off-by: Ming Lei <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

tools/testing/selftests/ublk/test_common.sh

···
 	local kill_server=$3
 
 	fio --name=job1 --filename=/dev/ublkb"${dev_id}" --ioengine=libaio \
-		--rw=readwrite --iodepth=256 --size="${size}" --numjobs=4 \
+		--rw=randrw --norandommap --iodepth=256 --size="${size}" --numjobs="$(nproc)" \
 		--runtime=20 --time_based > /dev/null 2>&1 &
 	sleep 2
 	if [ "${kill_server}" = "yes" ]; then

tools/testing/selftests/ublk/test_stress_02.sh

···
 _create_backfile 1 128M
 _create_backfile 2 128M
 
-ublk_io_and_kill_daemon 8G -t null -q 4 &
-ublk_io_and_kill_daemon 256M -t loop -q 4 "${UBLK_BACKFILES[0]}" &
-ublk_io_and_kill_daemon 256M -t stripe -q 4 "${UBLK_BACKFILES[1]}" "${UBLK_BACKFILES[2]}" &
-wait
+for nr_queue in 1 4; do
+	ublk_io_and_kill_daemon 8G -t null -q "$nr_queue" &
+	ublk_io_and_kill_daemon 256M -t loop -q "$nr_queue" "${UBLK_BACKFILES[0]}" &
+	ublk_io_and_kill_daemon 256M -t stripe -q "$nr_queue" "${UBLK_BACKFILES[1]}" "${UBLK_BACKFILES[2]}" &
+	wait
+done
 
 _cleanup_test "stress"
 _show_result $TID $ERR_CODE