ipv6: don't complain when deleting an address with prefix length of 128

Save prefix length in unused field in6_ifaddr->ia_plen, then on remove
check if an address has 128 prefix length, and if so, we don't need to
complain that there is none of related prefixes.

Reviewed by: kp
Obtained from: Yandex LLC
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D52952

(cherry picked from commit c4cce0a3e94e3b22f874508d1a9cd81442648238)