net/mlx5e: xsk: Fix DMA and xdp_frame leak on XDP_TX xmit failure

net/mlx5e: xsk: Fix DMA and xdp_frame leak on XDP_TX xmit failure

In the XSK branch of mlx5e_xmit_xdp_buff(), when sq->xmit_xdp_frame()
returns false (e.g. XDPSQ is full), the function returns without
unmapping the DMA address or freeing the xdp_frame allocated by
xdp_convert_zc_to_xdp_frame(). The xdpi_fifo push only happens on
success, so the completion path cannot recover these entries.

With CONFIG_DMA_API_DEBUG=y, the leak surfaces on driver unbind:

  DMA-API: pci 0000:08:00.0: device driver has pending DMA
  allocations while released from device [count=1116]
  One of leaked entries details: [device address=0x000000010ffd7028]
  [size=1534 bytes] [mapped with DMA_TO_DEVICE] [mapped as phy]
  WARNING: kernel/dma/debug.c:881 at dma_debug_device_change+0x127/0x180
  ...
  DMA-API: Mapped at:
   debug_dma_map_phys+0x4b/0xd0
   dma_map_phys+0xfd/0x2d0
   mlx5e_xdp_handle+0x5ae/0xac0 [mlx5_core]
   mlx5e_xsk_skb_from_cqe_mpwrq_linear+0xc4/0x170 [mlx5_core]
   mlx5e_handle_rx_cqe_mpwrq+0xc1/0x290 [mlx5_core]

Add the missing unmap + xdp_return_frame, matching the cleanup already
done in mlx5e_xdp_xmit(). has_frags is rejected earlier in this branch,
so no per-frag unmap is needed.

Fixes: 84a0a2310d6d ("net/mlx5e: XDP_TX from UMEM support")
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260604135446.456119-1-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>