In order to query tm info of nodes, priority and qset
for debugging, adds three debugfs files tm_nodes,
tm_priority and tm_qset in newly created tm directory.

Unlike previous debugfs commands, these three files
just support read ops, so they only support to use cat
command to dump their info.

The new tm file style is acccording to suggestion from
Jakub Kicinski's opinion as link https://lkml.org/lkml/2020/9/29/2101.
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>