echoRespondToAll: filter.Accept rather than filter.Drop on a match.

This function is only called in fake mode, which won't do anything more
with the packet after we respond to it anyway, so dropping it in the
prefilter is not necessary. And it's kinda semantically wrong: we did
not reject it, so telling the upper layer that it was rejected produces
an ugly error message.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

wgengine/userspace.go

@@ -374,10 +374,15 @@ func echoRespondToAll(p *packet.ParsedPacket, t *tstun.TUN) filter.Response {
 	if p.IsEchoRequest() {
 		header := p.ICMPHeader()
 		header.ToResponse()
-		packet := packet.Generate(&header, p.Payload())
-		t.InjectOutbound(packet)
-		// We already handled it, stop.
-		return filter.Drop
+		outp := packet.Generate(&header, p.Payload())
+		t.InjectOutbound(outp)
+		// We already responded to it, but it's not an error.
+		// Proceed with regular delivery. (Since this code is only
+		// used in fake mode, regular delivery just means throwing
+		// it away. If this ever gets run in non-fake mode, you'll
+		// get double responses to pings, which is an indicator you
+		// shouldn't be doing that I guess.)
+		return filter.Accept
 	}
 	return filter.Accept
 }