udp: fix endianness for port

[wadey]

If the host OS is already big endian, we were swapping bytes when we shouldn't have. Use the Go helper to make sure we do the endianness correctly

Fixes: #1189

[JackDoanRivian]

This made me curious how golang itself was handling portnumber-endianness, so I peeked into syscall_linux.go, this is very similar to what they do:

func (sa *SockaddrInet4) sockaddr() (unsafe.Pointer, _Socklen, error) {
	if sa.Port < 0 || sa.Port > 0xFFFF {
		return nil, 0, EINVAL
	}
	sa.raw.Family = AF_INET
	p := (*[2]byte)(unsafe.Pointer(&sa.raw.Port))
	p[0] = byte(sa.Port >> 8)
	p[1] = byte(sa.Port)
	sa.raw.Addr = sa.Addr
	return unsafe.Pointer(&sa.raw), SizeofSockaddrInet4, nil
}

except the use of binary.Bigendian makes it nice and obvious what's happening.

[wadey]

This made me curious how golang itself was handling portnumber-endianness, so I peeked into syscall_linux.go, this is very similar to what they do:

@JackDoanRivian I still don't understand how that works if the host OS is big or little endian, it seems like they don't do anything to handle that? I'm still confused about that. It does also make me wonder how our code in 1.8 also worked.

[wadey]

I still don't understand how that works if the host OS is big or little endian

ah ok I understand it now