CVE-2024-26739 net/sched: act_mirred: don't override retval if we already lost the skb

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

net/sched: act_mirred: don’t override retval if we already lost the skb

If we’re redirecting the skb, and haven’t called tcf_mirred_forward(),
yet, we need to tell the core to drop the skb by setting the retcode
to SHOT. If we have called tcf_mirred_forward(), however, the skb
is out of our hands and returning SHOT will lead to UaF.

Move the retval override to the error path which actually need it.