routing: add rib_add_default_route() wrapper

Multiple consumers in the kernel space want to install IPv4 or IPv6
default route. Provide convenient wrapper to simplify the code
inside the customers.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D36167

(cherry picked from commit d8b2693414ae5cebdbd4c64c9559275c4baff11e)