Prepare to make root overlay the default.

We plan on making --overlay2=root:self the default for runsc. That will be a
risky change which might need rollbacks. This change is in preparation for
that. We manually set --overlay2=none in places where we don't want the
overlay configuration to impact. This change should be a noop. The intention
for this change is to make the risky change very small and limited to just
flipping a flag.

PiperOrigin-RevId: 513930702

pkg/sentry/fsimpl/tmpfs/save_restore.go

@@ -17,7 +17,7 @@ package tmpfs
 // afterLoad is called by stateify.
 func (fs *filesystem) afterLoad() {
 	if fs.privateMF {
-		// TODO(b/241832602): Add S/R support.
+		// TODO(b/271612187): Add S/R support.
 		panic("S/R not supported for private memory files")
 	}
 	fs.mf = fs.mfp.MemoryFile()