in6_fib: return nh_ia in the ext interface as we do for IPv4

Like for IPv4 add nh_ia to the ext interface and return rt_ifa
in order to be used for, e.g., packet/octets accounting purposes.

Reviewed by: melifaro
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D23873