chore: disable fallback mode of tfo dialer

tfo-go caches the "unsupported" status when fallback mode is enabled.
In other words, if the hysteria server is started with
net.ipv4.tcp_fastopen=0 and it fails once, the tfo will not be enabled
until it is restarted, even if the user later sets sysctl
net.ipv4.tcp_fastopen=3.

extras/outbounds/fastopen.go

@@ -15,8 +15,7 @@ type fastOpenDialer struct {
 func newFastOpenDialer(netDialer *net.Dialer) *fastOpenDialer {
 	return &fastOpenDialer{
 		dialer: &tfo.Dialer{
-			Dialer:   *netDialer,
-			Fallback: true,
+			Dialer: *netDialer,
 		},
 	}
 }