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>

net/batching/conn_default.go

@@ -6,11 +6,12 @@
 package batching
 
 import (
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/types/nettype"
 )
 
 // TryUpgradeToConn is no-op on all platforms except linux.
-func TryUpgradeToConn(pconn nettype.PacketConn, _ string, _ int, _ string) nettype.PacketConn {
+func TryUpgradeToConn(pconn nettype.PacketConn, _ string, _ int, _ string, _ *controlknobs.Knobs) nettype.PacketConn {
 	return pconn
 }