in6: Add a helper function to compute expiry times

Tidy up a bunch of places that have the same duplicated logic. Simplify
callers of in6_init_prefix_ltimes(). No functional change intended.

Reviewed by: pouria, zlei, tuexen, glebius
MFC after: 2 weeks
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54561