overlay/tio: log dropped tun reads with bad virtio headers

decodeRead failures were silently swallowed; a kernel emitting an
unnegotiated GSO type would blackhole all tun traffic with nothing in
the logs. Debug-gated per the usual idiom so the happy path pays
nothing, which means plumbing the logger down through the offload
queueset.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ugV2edVqoz3tBvq9J6yWp

overlay/tun_linux.go

@@ -231,7 +231,7 @@ func newTunGeneric(c *config.C, l *slog.Logger, fd int, vnetHdr bool, offloadFla
 	var qs tio.QueueSet
 	var err error
 	if vnetHdr {
-		qs, err = tio.NewOffloadQueueSet(offloadUSOEnabled(offloadFlags))
+		qs, err = tio.NewOffloadQueueSet(offloadUSOEnabled(offloadFlags), l)
 	} else {
 		qs, err = tio.NewPollQueueSet()
 	}