Please remove assert from library code

[cathay4t]

The traffic_control folder has many assert, please replace them with fn XXX() -> Result<(), Error::InvalidNla>

[wllenyj]

The use of assert was to prevent the user from making mistakes and using the wrong API, such as calling ingress().egress(), ingress and egress are different Requests, Serial calls to ingress().egress() are wrong usage.

If there is a better way to achieve it is welcome.

[cathay4t]

Since this is crate/library, panic is never acceptable, we should never expect our user to catch panic. The Result<> is the only way a library/crate should used for failures.

If you want to assert user's input, please do that in the program using rtnetlink, not within, we are not CLI.