netinet6: embed struct nd_ifinfo into struct in6_ndireq

There is no functional change here, but we'd like to emphasize that the
nd_ifinfo structure is not a actually a kernel ND6 software context,
despite being actively used like this way, but an API/ABI structure for
ioctl(2). This should prevent from a ABI breakages like in 31ec8b6407fd.
This also is a step towards stopping using it as a kernel software
context.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D54722