netinet6/nd6: Sprinkle missing prefix refcounting

When we drop the prefix lock to call nd6_prefix_offlink() or
nd6_prefix_onlink(), make sure to keep the correpsonding prefix
structure alive. It is possible for a concurrent nd6_timer() to expire
the prefix while the lock is dropped.

Reported by: Maik Muench of Secfault Security
Reviewed by: pouria, zlei
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58423