[vfs2] Add FilesystemType.Release to avoid reference leaks.

Singleton filesystem like devpts and devtmpfs have a single filesystem shared
among all mounts, so they acquire a "self-reference" when initialized that
must be released when the entire virtual filesystem is released at sandbox
exit.

PiperOrigin-RevId: 336828852

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -81,6 +81,9 @@ func (FilesystemType) Name() string {
 	return Name
 }
 
+// Release implements vfs.FilesystemType.Release.
+func (FilesystemType) Release(ctx context.Context) {}
+
 // FilesystemOpts is used to pass configuration data to tmpfs.
 //
 // +stateify savable