The various Is methods (IsPrivate, IsLoopback, etc) did not work as expected for
IPv4-mapped IPv6 addresses, returning false for addresses which would return
true in their traditional IPv4 forms.

Thanks to Enze Wang of Alioth (@zer0yu) and Jianjun Chen of Zhongguancun Lab
(@chenjj) for reporting this issue.

This is CVE-2024-24790.

/cc @golang/security and @golang/release