Fix gateway setup for the interface routes.

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