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

[ae]

Summary

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.

Test Plan

ifconfig lo1 create
ifconfig lo1 inet6 fd00::1/128
ifconfig lo1 inet6 fd00::1/128 -alias

kernel: in6_unlink_ifa: autoconf'ed address fd00::1 has no prefix