Plug an ifaddr leak when changing a route's src

[rstone]

Summary

If a route is modified in a way that changes the route's source
address (i.e. the address used to access the gateway), then a
reference on the ifaddr representing the old source address will
be leaked if the address type does not have an ifa_rtrequest
method defined. Plug the leak by releasing the reference in
all cases.

Test Plan

I wrote this script to demonstrate the bug:

https://people.freebsd.org/~rstone/route-change-leak

On a stock -head system, I see the number of allocated entries in the ifaddr malloc type go up 100 every time I execute the script. With the fix, the number of allocated entries remains stable as expected.

[ae]

It would be nice if you describe why this leak happens, i.e. where leaked reference was acquired.

[rstone]

In D13417#281628, @ae wrote:

It would be nice if you describe why this leak happens, i.e. where leaked reference was acquired.

The reference is acquired at line 1800 in this file.

[ae]

From quick look, it seems ifa_rtrequest does not release this reference and the change is correct.