wgengine/magicsock: actually use AF_PACKET socket for raw disco

Previously, despite what the commit said, we were using a raw IP socket
that was *not* an AF_PACKET socket, and thus was subject to the host
firewall rules. Switch to using a real AF_PACKET socket to actually get
the functionality we want.

Updates #13140

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: If657daeeda9ab8d967e75a4f049c66e2bca54b78

net/packet/packet.go

@@ -393,6 +393,11 @@ func (q *Parsed) Buffer() []byte {
 // Payload returns the payload of the IP subprotocol section.
 // This is a read-only view; that is, q retains the ownership of the buffer.
 func (q *Parsed) Payload() []byte {
+	// If the packet is truncated, return nothing instead of crashing.
+	if q.length > len(q.b) || q.dataofs > len(q.b) {
+		return nil
+	}
+
 	return q.b[q.dataofs:q.length]
 }