PR 219829 Handle device removal and removal+add cases to fix infinity rate.

[ota_j.email.ne.jp]

Summary

The problem happens when a device is 1. removed, 2. removed and added back,
and 3. removed and a different one is added.

The case 2 applies to PR 219829 case.
One can reproduce the problem by creating, using, and destroying wlan or
tap devises as these are the easy one to add and remove on the fly.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=219829

Test Plan

Set up an interface that can be destroyed like tap or wlan.
% systat -if 1
% ifconfig wlan0 destroy
% ifconfig ... etc to recreate wlan0

This makes the some devices' peak too high.

[bcr]

OK from manpages.

[ota_j.email.ne.jp]

Drop unnecessary NULL check. This is a field on a struct and won't be NULL.

[ota_j.email.ne.jp]

Added static to implementatoin; follow style of other existing static functions.

[f20190109_eeeit.de]

Looks good.
I'll do some runtime tests before committing.

[mr]

Used wrong user for accepting...