The refcount of a CB buffer is initialized when user allocates a CB,
and is decreased when he destroys the CB handle.

If this refcount is increased also from kernel and user sends more than
one destroy requests for the handle, the buffer will be released/freed
and later be accessed when the refcount is put from kernel side.

To avoid it, prevent user from destroying the handle more than once.
Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>