drm/gem-shmem: Return vm_fault_t from drm_gem_shmem_try_map_pmd()

drm/gem-shmem: Return vm_fault_t from drm_gem_shmem_try_map_pmd()

Return the exact VM_FAULT_ mask from drm_gem_shmem_try_map_pmd(). Gives
the caller better insight into the result. Return 0 if nothing was done.

If the caller sees VM_FAULT_NOPAGE, drm_gem_shmem_try_map_pmd() added a
PMD entry to the page table. As before, return early from the page-fault
handler in that case.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patch.msgid.link/20260227114509.165572-4-tzimmermann@suse.de