net/sched: act_mirred: fix wrong device for mac_header_xmit check in tcf_blockcast_redir

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

net/sched: act_mirred: fix wrong device for mac_header_xmit check in tcf_blockcast_redir

In tcf_blockcast_redir(), when iterating block ports to redirect packets to multiple devices, the mac_header_xmit flag is queried from the wrong device. The loop sends to dev_prev but queries dev_is_mac_header_xmit(dev) — which is the NEXT device in the iteration, not the one being sent to.

This causes tcf_mirred_to_dev() to make incorrect decisions about whether to push or pull the MAC header. When the block contains mixed device types (e.g., an ethernet veth and a tunnel device), intermediate devices get the wrong mac_header_xmit flag, leading to skb header corruption. In the worst case, skb_push_rcsum with an incorrect mac_len can exhaust headroom and panic.

The last device in the loop is handled correctly (line 365-366 uses dev_is_mac_header_xmit(dev_prev)), confirming this is a copy-paste oversight for the intermediate devices.

Fix by using dev_prev instead of dev for the mac_header_xmit query, consistent with the device actually being sent to.