CVE-2024-42108 net: rswitch: Avoid use-after-free in rswitch_poll()

In the Linux kernel, the following vulnerability has been resolved:

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.