drm/gem: Introduce drm_gem_get_unmapped_area() fop

mmap() calls on the DRM file pointer currently always end up using
mm_get_unmapped_area() to get a free mapping region. On builds with
CONFIG_TRANSPARENT_HUGEPAGE enabled, this isn't ideal for GEM objects
backed by shmem buffers on mountpoints setting the 'huge=' option
because it can't correctly figure out the potentially huge address
alignment required.

This commit introduces the drm_gem_get_unmapped_area() function which
is meant to be used as a get_unmapped_area file operation on the DRM
file pointer to lookup GEM objects based on their fake offsets and get
a properly aligned region by calling shmem_get_unmapped_area() with
the right file pointer. If a GEM object isn't available at the given
offset or if the caller isn't granted access to it, the function falls
back to mm_get_unmapped_area().

This also makes drm_gem_get_unmapped_area() part of the default GEM
file operations so that all the DRM drivers can benefit from more
efficient mappings thanks to the huge page fault handler introduced in
previous commit 'drm/shmem-helper: Add huge page fault handler'.

The shmem_get_unmapped_area() function needs to be exported so that
it can be used from the DRM subsystem.

v3:
- include <linux/sched/mm.h> in drm_gem.c
- forward to shmem layer in builds with CONFIG_TRANSPARENT_HUGEPAGE=n

v6:
- use GPL variant to export drm_gem_get_unmapped_area()
- don't export shmem_get_unmapped_area() anymore (use f_op instead)

v11:
- rename drm_gem_object_lookup_from_offset() to
  drm_gem_object_lookup_at_offset()
- add Boris R-b

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patch.msgid.link/20251205182231.194072-4-loic.molinari@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

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