net: xilinx: axienet: Free outstanding DMA buffers on dmaengine stop

In the dmaengine path the driver pre-submits RX buffers and holds
in-flight TX buffers whose SKBs are DMA-mapped by the driver and freed
only in the completion callbacks. On ndo_stop() dmaengine_terminate_sync()
aborts these descriptors without running their callbacks, and the driver
then frees only the ring shells, leaking every SKB still owned by the
engine and its DMA mapping on each ifdown. With 128 RX buffers pre-posted
per channel, the mapping leak can eventually exhaust a limited IOMMU
aperture.

Clear the slot's skb in the TX and RX callbacks so a non-NULL skb marks a
slot that still owns a live, DMA-mapped buffer, and on stop unmap and free
every such buffer.

axienet_dma_rx_cb() runs from the DMA tasklet and re-arms the RX ring on
each completion, so it can race axienet_stop(): a completion may submit a
fresh buffer after dmaengine_terminate_sync() has returned, leaving the
channel armed with a buffer the teardown then frees while the engine may
still write into it (dma_release_channel() does not stop it either). Add a
lock that axienet_dma_rx_cb() holds across the @stopping check and the
resubmit, and axienet_stop() holds to set @stopping before terminating.
Once @stopping is set no callback can arm a new buffer, and any armed just
before is aborted by the terminate, so teardown only frees buffers the
engine no longer owns.

Fixes: 6a91b846af85 ("net: axienet: Introduce dmaengine support")
Cc: stable@vger.kernel.org
Signed-off-by: Suraj Gupta <suraj.gupta2@amd.com>
Link: https://patch.msgid.link/20260928184207.2361931-1-suraj.gupta2@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>