Kernel warning via invalid DMA unmapping

Improper DMA unmapping in the Linux kernel allows local users to trigger a kernel warning by deregistering an mlx5 device-memory MR. In mlx5_free_priv_descs(), a device-memory MR without an associated umem follows the ordinary-memory branch, causing dma_unmap_single() to process an address that was never mapped. The path requires local low-privilege access to an mlx5 RDMA device with device memory; the documented consequence is a kernel warning.