control/controlknobs,net/{batching,tstun},wgengine: add nodecaps to disable UDP & TUN GRO/GSO

Add four control-plane node attributes that let us disable UDP GSO/GRO
on the magicsock UDP socket and UDP/TCP GRO on the Tailscale TUN
device.

These complement the pre-existing TS_DEBUG_DISABLE_UDP_{GRO,GSO} and
TS_TUN_DISABLE_{UDP,TCP}_GRO envknobs. They exist so we can mitigate
upstream Linux kernel regressions on a deployed fleet without
requiring a client release, after two incidents (#13041, #19777) where
buggy kernel patches landed upstream and the fix took an excessively
long time to reach downstream distros.

Knob changes are reacted to in setNetworkMapInternal / SetNetworkMap via
a comparison against a cached "last applied" value and only an actual
transition triggers work: magicsock Rebind()+ReSTUN for UDP,
ApplyGROKnobs for TUN. The TUN side is gated by buildfeatures.HasGRO and
is one-way (wireguard-go GRO disablement is sticky); re-enabling
requires a client restart.

Updates #13041
Updates #19777

Change-Id: I802993070afa659cc06809bb0bfbb7f8a0cdb273
Signed-off-by: James Tucker <james@tailscale.com>

feature/relayserver/relayserver.go

@@ -69,7 +69,7 @@ func servePeerRelayDebugSessions(h *localapi.Handler, w http.ResponseWriter, r *
 func newExtension(logf logger.Logf, sb ipnext.SafeBackend) (ipnext.Extension, error) {
 	e := &extension{
 		newServerFn: func(logf logger.Logf, port uint16, onlyStaticAddrPorts bool) (relayServer, error) {
-			return udprelay.NewServer(logf, port, onlyStaticAddrPorts, sb.Sys().UserMetricsRegistry())
+			return udprelay.NewServer(logf, port, onlyStaticAddrPorts, sb.Sys().UserMetricsRegistry(), sb.Sys().ControlKnobs())
 		},
 		logf: logger.WithPrefix(logf, featureName+": "),
 	}