Changes from v4:
- check both vm_ops and vm_private_data to avoid (unlikely) confusion
with some other vma in map_vdso_once (as Andy noticed) - which would
lead to unable to use this API in that unlikely-case
(vm_private_data may be uninitialized and be the same as vvar_mapping
or vdso_mapping pointer) - so I introduced one-liner helper
vma_is_special_mapping().
Changes from v3:
- proper ifdefs around vdso_image_32
- missed Reviewed-by tag