net: rswitch: Avoid use-after-free in rswitch_poll()

net: rswitch: Avoid use-after-free in rswitch_poll()

The use-after-free is actually in rswitch_tx_free(), which is inlined in
rswitch_poll(). Since `skb` and `gq->skbs[gq->dirty]` are in fact the
same pointer, the skb is first freed using dev_kfree_skb_any(), then the
value in skb->len is used to update the interface statistics.

Let's move around the instructions to use skb->len before the skb is
freed.

This bug is trivial to reproduce using KFENCE. It will trigger a splat
every few packets. A simple ARP request or ICMP echo request is enough.

Fixes: 271e015b9153 ("net: rswitch: Add unmap_addrs instead of dma address in each desc")
Signed-off-by: Radu Rendec <rrendec@redhat.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://patch.msgid.link/20240702210838.2703228-1-rrendec@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>