ifaddr reference count leaks that seem to be related to routing code

[Ryan Stone]

Thanks!  For reference, I tried this patch and it did resolve my leak:

https://github.com/rysto32/freebsd/commit/73caa71c351c072d673d477972fda2aeb369eb3d

I haven't exhaustively tested the routing code, though, so I can't say
for certain that the assert will always be true, nor am I certain that
no callers were depending on ifa_ref by rt_getifa_fib().