tests: return /dev/ublkb-unknown if __create_ublk_dev() fails

This way can make the following test failed easily.

Signed-off-by: Ming Lei <[email protected]>

tests/common/fio_common

@@ -240,7 +240,11 @@ __create_ublk_dev()
 	[ ${id} == "-" ] && echo "no free ublk device nodes" && exit -1
 	eval $UBLK add ${T_TYPE_PARAMS} -n $id > /dev/null 2>&1
 	udevadm settle
-	echo "/dev/ublkb${id}"
+	if [ -b /dev/ublkb0 ]; then
+		echo "/dev/ublkb${id}"
+	else
+		echo "/dev/ublkb-unknown"
+	fi
 }
 
 __recover_ublk_dev()