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>

net/neterror/neterror.go

@@ -6,6 +6,7 @@ package neterror
 
 import (
 	"errors"
+	"fmt"
 	"runtime"
 	"syscall"
 )
@@ -57,3 +58,25 @@ func PacketWasTruncated(err error) bool {
 	}
 	return packetWasTruncated(err)
 }
+
+var shouldDisableUDPGSO func(error) bool // non-nil on Linux
+
+func ShouldDisableUDPGSO(err error) bool {
+	if shouldDisableUDPGSO == nil {
+		return false
+	}
+	return shouldDisableUDPGSO(err)
+}
+
+type ErrUDPGSODisabled struct {
+	OnLaddr  string
+	RetryErr error
+}
+
+func (e ErrUDPGSODisabled) Error() string {
+	return fmt.Sprintf("disabled UDP GSO on %s, NIC(s) may not support checksum offload", e.OnLaddr)
+}
+
+func (e ErrUDPGSODisabled) Unwrap() error {
+	return e.RetryErr
+}