feature/conn25: drop returned error from NewFlow signature

The returned error in the signature is left over from previous
implementations and was only returning nil.

If we know NewFlow will succeed we can fire a create hook (implemented
in a future commit) before NewFlow, which will prevent a remove hook for
a flow from firing before the create hook for the same flow.

Updates tailscale/corp#38630

Signed-off-by: Michael Ben-Ami <mzb@tailscale.com>

feature/conn25/datapath.go

@@ -175,13 +175,11 @@ func (dh *datapathHandler) HandlePacketFromWireGuard(p *packet.Parsed, tun *tstu
 		Tuple:  flowtrack.MakeTuple(p.IPProto, netip.AddrPortFrom(realIP, p.Dst.Port()), p.Src),
 		Action: dh.snatAction(transitIP),
 	}
-	if err := dh.connectorFlowTable.NewFlow(FlowData{
+	dh.connectorFlowTable.NewFlow(FlowData{
 		FromTun: incoming,
 		FromWG:  outgoing,
-	}); err != nil {
-		dh.debugLogf("error installing flow, passing packet unmodified: %v", err)
-		return filter.Accept
-	}
+	})
+
 	outgoing.Action(p)
 	return filter.Accept
 }
@@ -244,13 +242,11 @@ func (dh *datapathHandler) HandlePacketFromTunDevice(p *packet.Parsed) filter.Re
 		Tuple:  flowtrack.MakeTuple(p.IPProto, netip.AddrPortFrom(transitIP, p.Dst.Port()), p.Src),
 		Action: dh.snatAction(magicIP),
 	}
-	if err := dh.clientFlowTable.NewFlow(FlowData{
+	dh.clientFlowTable.NewFlow(FlowData{
 		FromTun: outgoing,
 		FromWG:  incoming,
-	}); err != nil {
-		dh.debugLogf("error installing flow from tun device, passing packet unmodified: %v", err)
-		return filter.Accept
-	}
+	})
+
 	outgoing.Action(p)
 	return filter.Accept
 }