envknob: support changing envknobs post-init

Updates #5114

Change-Id: Ia423fc7486e1b3f3180a26308278be0086fae49b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tun.go

@@ -20,14 +20,6 @@ import (
 	"tailscale.com/types/logger"
 )
 
-var tunMTU = DefaultMTU
-
-func init() {
-	if mtu, ok := envknob.LookupInt("TS_DEBUG_MTU"); ok {
-		tunMTU = mtu
-	}
-}
-
 // createTAP is non-nil on Linux.
 var createTAP func(tapName, bridgeName string) (tun.Device, error)
 
@@ -52,6 +44,10 @@ func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
 		}
 		dev, err = createTAP(tapName, bridgeName)
 	} else {
+		tunMTU := DefaultMTU
+		if mtu, ok := envknob.LookupInt("TS_DEBUG_MTU"); ok {
+			tunMTU = mtu
+		}
 		dev, err = tun.CreateTUN(tunName, tunMTU)
 	}
 	if err != nil {