ND6: Set the correct state for new neighbor cache entries

Restore state 6. Many of the UNH tests end up exercising this
state, where we have a new neighbor cache entry and a new link-layer
entry is being created for it. The link-layer address is currently
unknown so the initial state of the "llentry" should remain initialized
to ND6_LLINFO_NOSTATE so that the ND code will send a solicitation.
Setting this to ND6_LLINFO_STALE implies that the link-level entry
is valid and can be used (but needs to be refreshed via the Neighbor
Unreachability state machine).

https://forums.freebsd.org/threads/64287/

Submitted by: Farrell Woods <Farrell_Woods@Dell.com>
Reviewed by: mjoras, dab, ae
MFC after: 1 week
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D14059