wgengine: predict min.Peers length across calls

The number of peers we have will be pretty stable across time.
Allocate roughly the right slice size.
This reduces memory usage when there are many peers.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/userspace.go

@@ -108,6 +108,7 @@ type userspaceEngine struct {
 
 	wgLock              sync.Mutex // serializes all wgdev operations; see lock order comment below
 	lastCfgFull         wgcfg.Config
+	lastNMinPeers       int
 	lastRouterSig       deephash.Sum // of router.Config
 	lastEngineSigFull   deephash.Sum // of full wireguard config
 	lastEngineSigTrim   deephash.Sum // of trimmed wireguard config
@@ -606,7 +607,7 @@ func (e *userspaceEngine) maybeReconfigWireguardLocked(discoChanged map[key.Publ
 	// based on the full config. Prune off all the peers
 	// and only add the active ones back.
 	min := full
-	min.Peers = nil
+	min.Peers = make([]wgcfg.Peer, 0, e.lastNMinPeers)
 
 	// We'll only keep a peer around if it's been active in
 	// the past 5 minutes. That's more than WireGuard's key
@@ -650,6 +651,7 @@ func (e *userspaceEngine) maybeReconfigWireguardLocked(discoChanged map[key.Publ
 			trimmedDisco[dk] = true
 		}
 	}
+	e.lastNMinPeers = len(min.Peers)
 
 	if !deephash.Update(&e.lastEngineSigTrim, &min, trimmedDisco, trackDisco, trackIPs) {
 		// No changes