ensure device private pages have an owner

[Christoph Hellwig]

When acting on device private mappings a driver needs to know if the
device (or other entity in case of kvmppc) actually owns this private
mapping.  This series adds an owner field and converts the migrate_vma
code over to check it.  I looked into doing the same for
hmm_range_fault, but as far as I can tell that code has never been
wired up to actually work for device private memory, so instead of
trying to fix some unused code the second patch just remove the code.
We can add it back once we have a working and fully tested code, and
then should pass the expected owner in the hmm_range structure.