drm/shmem_helper: Make sure PMD entries get the writeable upgrade

drm/shmem_helper: Make sure PMD entries get the writeable upgrade

Unlike PTEs which are automatically upgraded to writeable entries if
.pfn_mkwrite() returns 0, the PMD upgrades go through .huge_fault(),
and we currently pretend to have handled the make-writeable request
even though we only ever map things read-only. Make sure we pass the
proper "write" info to vmf_insert_pfn_pmd() in that case.

This also means we have to record the mkwrite event in the .huge_fault()
path now. Move the dirty tracking logic to a
drm_gem_shmem_record_mkwrite() helper so it can also be called from
drm_gem_shmem_pfn_mkwrite().

Note that this wasn't a problem before commit 28e3918179aa
("drm/gem-shmem: Track folio accessed/dirty status in mmap"), because
the pgprot were not lowered to read-only before this commit (see the
vma_wants_writenotify() in vma_set_page_prot()).

Fixes: 28e3918179aa ("drm/gem-shmem: Track folio accessed/dirty status in mmap")
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Reviewed-by: Loïc Molinari <loic.molinari@collabora.com>
Tested-by: Biju Das <biju.das.jz@bp.renesas.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
Link: https://patch.msgid.link/20260320151914.586945-1-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>