rc.d/routing: Correct setting default gateway for each FIB

There was a mistake in the previous commit, it used the incorrect
spelling of the FIB variable name and was not functional

Also corrects an issue with the IPv6 default route variable name.

Fixes: 30659d1dcbcc ("Add support for adding default routes for other FIBs")
Sponsored-by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D37685