mm/hmm: use device_private_entry_to_pfn()

mm/hmm: use device_private_entry_to_pfn()

swp_offset() should not be called directly, the wrappers are supposed to
abstract away the encoding of the device_private specific information in
the swap entry.

Link: https://lore.kernel.org/r/20200327200021.29372-7-jgg@ziepe.ca
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>