support TP_STATUS_CSUMNOTREADY

[M1cha]

Currently, the dhcp client doesn't work in (some) VMs, in my case a LXD container communicating through a linux bridge.
I'm using the raw_socket phy to test this.

The reason for that is that - while the IPv4 checksum is valid - the UDP checksum is not since it's expected to be ignored even though it's non-zero.

To detect this we'd have to use recvmsg instead of read inside RawSocketDesc::recv. Getting this information to where it's needed might be quite invasive though because we need a way to tell udp::Repr::parse to ignore the checksum for some packets.

References: https://github.com/mirror/busybox/blob/e512aeb0fb3c585948ae6517cfdf4a53cf99774d/networking/udhcp/dhcpc.c#L952

[Dirbaio]

Is the value in the checksum field not zero? If it's zero it should be accepted:

smoltcp/src/wire/udp.rs

Lines 118 to 124 in 1f7e1fa

	// From the RFC:
	// > An all zero transmitted checksum value means that the transmitter
	// > generated no checksum (for debugging or for higher level protocols
	// > that don't care).
	if self.checksum() == 0 {
	return true;
	}

If it's garbage instead of zero then I'd say this is a Linux bug...