Enable THP support in drm_pagemap

Use Balbir Singh's series for device-private THP support [1] and previous
preparation work in drm_pagemap [2] to add 2MB/THP support in xe. This leads
to significant performance improvements when using SVM with 2MB pages.

[1] https://lore.kernel.org/linux-mm/20251001065707.920170-1-...
[2] https://patchwork.freedesktop.org/series/151754/

v2:
- rebase on top of multi-device SVM
- add drm_pagemap_cpages() with temporary patch
- address other feedback from Matt Brost on v1

v3:
The major change is to remove the dependency to the mm/huge_memory
helper migrate_device_split_page() that was called explicitely when
a 2M buddy allocation backed by a large folio would be later reused
for a smaller allocation (4K or 64K). Instead, the first 3 patches
provided by Matthew Brost ensure large folios are split at the time
of freeing.

Francois Dugast (3):
  drm/pagemap: Unlock and put folios when possible
  drm/pagemap: Add helper to access zone_device_data
  drm/pagemap: Enable THP support for GPU memory migration

Matthew Brost (4):
  mm: Add folio_split_unref helper
  fs/dax: Use folio_split_unref helper
  mm: Split device-private and coherent folios before freeing
  drm/pagemap: Correct cpages calculation for migrate_vma_setup

 drivers/gpu/drm/drm_gpusvm.c  |   7 +-
 drivers/gpu/drm/drm_pagemap.c | 154 ++++++++++++++++++++++++++++------
 fs/dax.c                      |  25 +-----
 include/drm/drm_pagemap.h     |  15 ++++
 include/linux/huge_mm.h       |   1 +
 mm/huge_memory.c              |  39 +++++++++
 mm/memremap.c                 |   2 +
 7 files changed, 190 insertions(+), 53 deletions(-)

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2.43.0