net: fix WriteMsgUDPAddrPort addr handling

WriteMsgUDPAddrPort should accept IPv4 target addresses on IPv6 UDP sockets. An IPv4 target address will be converted to an IPv4-mapped IPv6 address. Fixes golang#52264.

src/net/ipsock_posix.go

@@ -215,8 +215,11 @@ func addrPortToSockaddrInet4(ap netip.AddrPort) (syscall.SockaddrInet4, error) {
 func addrPortToSockaddrInet6(ap netip.AddrPort) (syscall.SockaddrInet6, error) {
 	// ipToSockaddrInet6 has special handling here for zero length slices.
 	// We do not, because netip has no concept of a generic zero IP address.
+	//
+	// addr is allowed to be an IPv4 address, because As16 will convert it
+	// to an IPv4-mapped IPv6 address.
 	addr := ap.Addr()
-	if !addr.Is6() {
+	if !addr.Is6() && !addr.Is4() {
 		return syscall.SockaddrInet6{}, &AddrError{Err: "non-IPv6 address", Addr: addr.String()}
 	}
 	sa := syscall.SockaddrInet6{