so saves us
   some cycles since the page would be stuck in a resubmit-redirty loop
   until the fixup worker completes. It's not harmful, just wasteful.
 - If the fixup worker fails, we mark the page and mapping as errored,
   and end the writeback, similar to what we would do had the page
   actually been submitted to writeback.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>