virtio: console: Barrier needed before dropping early_put_chars?

Hey Rusty, Christian,

Christoph Hellwig asked why we don't need a barrier before this code in
virtcons_probe():

+     /* Start using the new console output. */
+     early_put_chars = NULL;
      return 0;

Since only s390 uses early_put_chars so far, you'd know why it's not
needed / why we're safe.

Thanks,
		Amit