net/mlx5e: Reuse per-RQ XDP buffer to avoid stack zeroing overhead

CONFIG_INIT_STACK_ALL_ZERO introduces a performance cost by
zero-initializing all stack variables on function entry. The mlx5 XDP
RX path previously allocated a struct mlx5e_xdp_buff on the stack per
received CQE, resulting in measurable performance degradation under
this config.

This patch reuses a mlx5e_xdp_buff stored in the mlx5e_rq struct,
avoiding per-CQE stack allocations and repeated zeroing.

With this change, XDP_DROP and XDP_TX performance matches that of
kernels built without CONFIG_INIT_STACK_ALL_ZERO.

Performance was measured on a ConnectX-6Dx using a single RX channel
(1 CPU at 100% usage) at ~50 Mpps. The baseline results were taken from
net-next-6.15.

Stack zeroing disabled:
- XDP_DROP:
    * baseline:                     31.47 Mpps
    * baseline + per-RQ allocation: 32.31 Mpps (+2.68%)

- XDP_TX:
    * baseline:                     12.41 Mpps
    * baseline + per-RQ allocation: 12.95 Mpps (+4.30%)

Stack zeroing enabled:
- XDP_DROP:
    * baseline:                     24.32 Mpps
    * baseline + per-RQ allocation: 32.27 Mpps (+32.7%)

- XDP_TX:
    * baseline:                     11.80 Mpps
    * baseline + per-RQ allocation: 12.24 Mpps (+3.72%)

Reported-by: Sebastiano Miano <mianosebastiano@gmail.com>
Reported-by: Samuel Dobron <sdobron@redhat.com>
Link: https://lore.kernel.org/all/CAMENy5pb8ea+piKLg5q5yRTMZacQqYWAoVLE1FE9WhQPq92E0g@mail.gmail.com/


Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://patch.msgid.link/1747253032-663457-1-git-send-email-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>