netgraph/ng_bridge: Document staleness in multithreaded operation

[donner]

Summary

In the data path of ng_bridge(4), the only value of an host struct,
which needs to be modified, is the staleness, which is reset every
time a frame was received. It's save to leave the code as it is.

Depends on D28545

Test Plan

No functional changes.

[kp]

I don't know if counter(9) makes sense here. That's usually intended for statistical information (e.g. how many packets passed through this interface). This looks like something that wants to be protected by a lock, or implemented with an atomic.

[donner]

In D28546#639111, @kp wrote:

I don't know if counter(9) makes sense here.

The only shared access to the variable is setting it to zero.
All other operations (like regular callouts and control message processing) already have a WRITER lock on the whole node.

So there is no need for any synchronization all.
The only change to make is to document this reasoning.

[donner]

• Revert "Switch staleness to counter framework."
• Comment why it's safe to do so.

[donner]

OTOH it's bad, because it prevents the code from running under const constraints.