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
MFC after: 1 week
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D52952