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/flowtable.go

@@ -196,7 +196,7 @@ func (t *FlowTable) lookup(k flowtrack.Tuple, dir Origin) (PacketAction, bool) {
 // would cause the table to exceed its maximum size, the least recently used
 // (looked-up or created) flow is evicted. data is not validated, the caller must
 // supply non-nil packet actions.
-func (t *FlowTable) NewFlow(data FlowData) error {
+func (t *FlowTable) NewFlow(data FlowData) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
 
@@ -217,8 +217,6 @@ func (t *FlowTable) NewFlow(data FlowData) error {
 
 	t.fromTunCache[data.FromTun.Tuple] = ele
 	t.fromWGCache[data.FromWG.Tuple] = ele
-
-	return nil
 }
 
 // StartExpiredSweeper starts a sweeper that removes idle flows that have