Currently, the debugfs command for dump mac tnl status is
implemented by "echo xxxx > cmd", and record the information
in dmesg. It's unnecessary and heavy. To improve it, create
a single file "mac_tnl_status" for it, and query it by command
"cat mac_tnl_status", return the result to userspace, rather
than record in dmesg.

The display style is below:
$ cat mac_tnl_status
Recently generated mac tnl interruption:
[0111204.175437] status = 0x30
[0154120.329912] status = 0x30
Signed-off-by: Jiaran Zhang <zhangjiaran@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>