Handle the driver KPI change from r292373. Ensure that managed device
pagers fault routines always return with a result page, be it the
proper and valid result page, or initially passed freshly allocated
placeholder. Do not free the passed in page until we are able to
provide the replacement, and do not assign NULL to *mres.
Reported and tested by: dumbbell
Reviewed by: royger (who also verified that Xen code is safe)
Sponsored by: The FreeBSD Foundation