[intfstat] Fix error code not define

[inspurSDN]

What I did

Fix undefined error code

How I did it

Import errno lib

How to verify it

Run unit tests

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[inspurSDN]

@lguohan could you help to review it?

[lguohan]

code has been refactored, do not see issue in the new code. class UserCache.