p54: avoid accessing the data mapped to streaming DMA

commit 47876285

 upstream.

In p54p_tx(), skb->data is mapped to streaming DMA on line 337:
  mapping = pci_map_single(..., skb->data, ...);

Then skb->data is accessed on line 349:
  desc->device_addr = ((struct p54_hdr *)skb->data)->req_id;

This access may cause data inconsistency between CPU cache and hardware.

To fix this problem, ((struct p54_hdr *)skb->data)->req_id is stored in
a local variable before DMA mapping, and then the driver accesses this
local variable instead of skb->data.

Cc: <stable@vger.kernel.org>
Signed-off-by: Jia-Ju Bai <baijiaju@tsinghua.edu.cn>
Acked-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200802132949.26788-1-baijiaju@tsinghua.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>