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>

wgengine/magicsock/rebinding_conn.go

@@ -12,6 +12,7 @@ import (
 	"syscall"
 
 	"golang.org/x/net/ipv6"
+	"tailscale.com/control/controlknobs"
 	"tailscale.com/net/batching"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/packet"
@@ -41,9 +42,9 @@ type RebindingUDPConn struct {
 // nettype.PacketConn to a batchingConn when appropriate. This upgrade is
 // intentionally pushed closest to where read/write ops occur in order to avoid
 // disrupting surrounding code that assumes nettype.PacketConn is a
-// *net.UDPConn.
-func (c *RebindingUDPConn) setConnLocked(p nettype.PacketConn, network string, batchSize int) {
-	upc := batching.TryUpgradeToConn(p, network, batchSize, "magicsock_udp_rxq_overflows")
+// *net.UDPConn. knobs may be nil.
+func (c *RebindingUDPConn) setConnLocked(p nettype.PacketConn, network string, batchSize int, knobs *controlknobs.Knobs) {
+	upc := batching.TryUpgradeToConn(p, network, batchSize, "magicsock_udp_rxq_overflows", knobs)
 	c.pconn = upc
 	c.pconnAtomic.Store(&upc)
 	c.port = uint16(c.localAddrLocked().Port)