selftests: ublk: don't take same backing file for more than one ublk devices

Don't use same backing file for more than one ublk devices, and avoid
concurrent write on same file from more ublk disks.

Fixes: 8ccebc1 ("selftests: ublk: support UBLK_F_AUTO_BUF_REG")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250623011934.741788-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

tools/testing/selftests/ublk/test_stress_03.sh

@@ -32,22 +32,23 @@ _create_backfile 2 128M
 ublk_io_and_remove 8G -t null -q 4 -z &
 ublk_io_and_remove 256M -t loop -q 4 -z "${UBLK_BACKFILES[0]}" &
 ublk_io_and_remove 256M -t stripe -q 4 -z "${UBLK_BACKFILES[1]}" "${UBLK_BACKFILES[2]}" &
+wait
 
 if _have_feature "AUTO_BUF_REG"; then
 	ublk_io_and_remove 8G -t null -q 4 --auto_zc &
 	ublk_io_and_remove 256M -t loop -q 4 --auto_zc "${UBLK_BACKFILES[0]}" &
 	ublk_io_and_remove 256M -t stripe -q 4 --auto_zc "${UBLK_BACKFILES[1]}" "${UBLK_BACKFILES[2]}" &
 	ublk_io_and_remove 8G -t null -q 4 -z --auto_zc --auto_zc_fallback &
+	wait
 fi
-wait
 
 if _have_feature "PER_IO_DAEMON"; then
 	ublk_io_and_remove 8G -t null -q 4 --auto_zc --nthreads 8 --per_io_tasks &
 	ublk_io_and_remove 256M -t loop -q 4 --auto_zc --nthreads 8 --per_io_tasks "${UBLK_BACKFILES[0]}" &
 	ublk_io_and_remove 256M -t stripe -q 4 --auto_zc --nthreads 8 --per_io_tasks "${UBLK_BACKFILES[1]}" "${UBLK_BACKFILES[2]}" &
 	ublk_io_and_remove 8G -t null -q 4 -z --auto_zc --auto_zc_fallback --nthreads 8 --per_io_tasks &
+	wait
 fi
-wait
 
 _cleanup_test "stress"
 _show_result $TID $ERR_CODE