drm/pagemap: Correct cpages calculation for migrate_vma_setup

drm/pagemap: Correct cpages calculation for migrate_vma_setup

cpages returned from migrate_vma_setup represents the total number of
individual pages found, not the number of 4K pages. The math in
drm_pagemap_migrate_to_devmem for npages is based on the number of 4K
pages, so cpages != npages can fail even if the entire memory range is
found in migrate_vma_setup (e.g., when a single 2M page is found).
Add drm_pagemap_cpages, which converts cpages to the number of 4K pages
found.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: linux-mm@kvack.org
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Balbir Singh <balbirs@nvidia.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260312192126.2024853-4-francois.dugast@intel.com