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: 26d3ac3c ("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