Currently rinit1() and its IPv6 counterpart nd6_prefix_onlink_rtrequest() uses dummy null_sdl gateway address during route insertion and fill it afterwards. One of the original reasons might reside in the concern that sizeof(struct sockadd_dl) is rather big to be on-stack (54 bytes on amd64).
Inserting fake gateway and silently fixing it afterwards brings complications to the routing stack.

This change fixes both rinit1() and nd6_prefix_onlink_rtrequest() by filling in proper gateway prior to the inserion. 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())