drm/shmem-helper: Reset vma->vm_ops before calling dma_buf_mmap()

The dma-buf backend is supposed to provide its own vm_ops, but some
implementation just have nothing special to do and leave vm_ops
untouched, probably expecting this field to be zero initialized (this
is the case with the system_heap implementation for instance).
Let's reset vma->vm_ops to NULL to keep things working with these
implementations.

Fixes: 26d3ac3cb0 ("drm/shmem-helpers: Redirect mmap for imported dma-buf")
Cc: <stable@vger.kernel.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reported-by: Roman Stratiienko <r.stratiienko@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Roman Stratiienko <r.stratiienko@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230724112610.60974-1-boris.brezillon@collabora.com

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -623,7 +623,13 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct
 	int ret;
 
 	if (obj->import_attach) {
+		/* Reset both vm_ops and vm_private_data, so we don't end up with
+		 * vm_ops pointing to our implementation if the dma-buf backend
+		 * doesn't set those fields.
+		 */
 		vma->vm_private_data = NULL;
+		vma->vm_ops = NULL;
+
 		ret = dma_buf_mmap(obj->dma_buf, vma, 0);
 
 		/* Drop the reference drm_gem_mmap_obj() acquired.*/