selftests: ublk: don't share backing files between ublk servers

stress_04 is missing a wait between blocks of tests, meaning multiple
ublk servers will be running in parallel using the same backing files.
Add a wait after each section to ensure each backing file is in use by a
single ublk server at a time.

Signed-off-by: Caleb Sander Mateos <[e-mail]>
Reviewed-by: Ming Lei <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

tools/testing/selftests/ublk/test_stress_04.sh

···
 ublk_io_and_kill_daemon 8G -t null -q 4 -z --no_ublk_fixed_fd &
 ublk_io_and_kill_daemon 256M -t loop -q 4 -z --no_ublk_fixed_fd "${UBLK_BACKFILES[0]}" &
 ublk_io_and_kill_daemon 256M -t stripe -q 4 -z "${UBLK_BACKFILES[1]}" "${UBLK_BACKFILES[2]}" &
+wait
 
 if _have_feature "AUTO_BUF_REG"; then
 	ublk_io_and_kill_daemon 8G -t null -q 4 --auto_zc &
 	ublk_io_and_kill_daemon 256M -t loop -q 4 --auto_zc "${UBLK_BACKFILES[0]}" &
 	ublk_io_and_kill_daemon 256M -t stripe -q 4 --auto_zc --no_ublk_fixed_fd "${UBLK_BACKFILES[1]}" "${UBLK_BACKFILES[2]}" &
 	ublk_io_and_kill_daemon 8G -t null -q 4 -z --auto_zc --auto_zc_fallback &
+	wait
 fi
 
 if _have_feature "PER_IO_DAEMON"; then
···
 	ublk_io_and_kill_daemon 256M -t loop -q 4 --auto_zc --nthreads 8 --per_io_tasks "${UBLK_BACKFILES[0]}" &
 	ublk_io_and_kill_daemon 256M -t stripe -q 4 --auto_zc --nthreads 8 --per_io_tasks "${UBLK_BACKFILES[1]}" "${UBLK_BACKFILES[2]}" &
 	ublk_io_and_kill_daemon 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