Create nd6_ns_output_fib() function with extra argument fibnum. Use it
to initialize mbuf's fibnum. Uninitialized fibnum value can lead to
panic in the routing code. Currently we use only RT_DEFAULT_FIB value
for initialization.

Differential Revision: https://reviews.freebsd.org/D1998
Reviewed by: hrs (previous version)
Sponsored by: Yandex LLC