net/neterror, wgengine/magicsock: use UDP GSO and GRO on Linux (#7791)

This commit implements UDP offloading for Linux. GSO size is passed to
and from the kernel via socket control messages. Support is probed at
runtime.

UDP GSO is dependent on checksum offload support on the egress netdev.
UDP GSO will be disabled in the event sendmmsg() returns EIO, which is
a strong signal that the egress netdev does not support checksum
offload.

Updates tailscale/corp#8734

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/magicsock_default.go

@@ -20,3 +20,21 @@ func (c *Conn) listenRawDisco(family string) (io.Closer, error) {
 func trySetSocketBuffer(pconn nettype.PacketConn, logf logger.Logf) {
 	portableTrySetSocketBuffer(pconn, logf)
 }
+
+func tryEnableUDPOffload(pconn nettype.PacketConn) (hasTX bool, hasRX bool) {
+	return false, false
+}
+
+func getGSOSizeFromControl(control []byte) (int, error) {
+	return 0, nil
+}
+
+func setGSOSizeInControl(control *[]byte, gso uint16) {}
+
+func errShouldDisableOffload(err error) bool {
+	return false
+}
+
+const (
+	controlMessageSize = 0
+)