hsr: add hsr root debugfs directory

In current hsr code, when hsr interface is created, it creates debugfs
directory /sys/kernel/debug/<interface name>.
If there is same directory or file name in there, it fails.
In order to reduce possibility of failure of creation of debugfs,
this patch adds root directory.

Test commands:
    ip link add dummy0 type dummy
    ip link add dummy1 type dummy
    ip link add hsr0 type hsr slave1 dummy0 slave2 dummy1

Before this patch:
    /sys/kernel/debug/hsr0/node_table

After this patch:
    /sys/kernel/debug/hsr/hsr0/node_table

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/hsr/hsr_main.c

@@ -123,6 +123,7 @@ static void __exit hsr_exit(void)
 {
 	unregister_netdevice_notifier(&hsr_nb);
 	hsr_netlink_exit();
+	hsr_debugfs_remove_root();
 }
 
 module_init(hsr_init);