cmd/tailscaled: disable netns in userspace-networking mode

Updates #2827
Updates #2822

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

cmd/tailscaled/tailscaled.go

@@ -32,6 +32,7 @@ import (
 	"tailscale.com/ipn/ipnserver"
 	"tailscale.com/logpolicy"
 	"tailscale.com/net/dns"
+	"tailscale.com/net/netns"
 	"tailscale.com/net/socks5/tssocks"
 	"tailscale.com/net/tstun"
 	"tailscale.com/paths"
@@ -296,6 +297,9 @@ func run() error {
 		logf("wgengine.New: %v", err)
 		return err
 	}
+	if useNetstack {
+		netns.Disable()
+	}
 
 	var ns *netstack.Impl
 	if useNetstack || wrapNetstack {