Generic `if_output` callback signature was modified to use `struct route`
instead of `struct rtentry` in [[ https://svnweb.freebsd.org/base?view=revision&revision=191148 | r191148 ]], back in 2009.
Quoting commit message:
```
Change if_output to take a struct route as its fourth argument in order
to allow passing a cached struct llentry * down to L2
```
Fix `bridge_output()` to match this signature and update the remaining comment in if_var.h.