Pass radix head ptr along with rte to rtexpunge().
Rename rtexpunge to rt_expunge().
Remove additional fib checks from rtalloc1_fib.
It looks like current consumers are either unaware
of MRT (and uses RT_DEFAULT_FIB implicitly) or
know what thay are doing, In latter case they
will be either hit by KASSERT or ESCRH will be returned
due to NULL rnh.