mm: handle multiple owners of device private pages in migrate_vma

Add a new src_owner field to struct migrate_vma.  If the field is set,
only device private pages with page->pgmap->owner equal to that field are
migrated.  If the field is not set only "normal" pages are migrated.

Fixes: df6ad698

 ("mm/device-public-memory: device memory cache coherent with CPU")
Link: https://lore.kernel.org/r/20200316193216.920734-3-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Tested-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>