Fix gateway setup for the interface routes.
Currently rinit1() and its IPv6 counterpart
nd6_prefix_onlink_rtrequest() uses dummy null_sdl gateway address during route insertion and change it afterwards. This behaviour brings complications to the routing stack and the users of its upcoming notification system.
This change fixes both rinit1() and nd6_prefix_onlink_rtrequest()
by filling in proper gateway in the beginning. It does not change any of the userland notifications as in both cases, they happen after the insertion and fixup process (rt_newaddrmsg_fib() and nd6_rtmsg()).
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20328