ensure device private pages have an owner v2

[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.

Changes since v1:
 - split out the pgmap->owner addition into a separate patch
 - check pgmap->owner is set for device private mappings
 - rename the dev_private_owner field in struct migrate_vma to src_owner
 - refuse to migrate private pages if src_owner is not set
 - keep the non-fault device private handling in hmm_range_fault