drm/gem-shmem: Don't store mmap'ed buffers in core dumps

Set the VM_DONTDUMP flag on mmap'ed VMAs to omit them from core
dumps. It's display-buffer memory; who knows what secrets these
buffers contain.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Chia-I Wu <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem_shmem_helper.c

···
 		return ret;
 	}
 
-	vma->vm_flags |= VM_PFNMAP | VM_DONTEXPAND;
+	vma->vm_flags |= VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
 	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 	if (shmem->map_wc)
 		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);