net/sched: act_skbmod: Skip non-Ethernet packets

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

net/sched: act_skbmod: Skip non-Ethernet packets

Currently tcf_skbmod_act() assumes that packets use Ethernet as their L2

protocol, which is not always the case. As an example, for CAN devices:

$ ip link add dev vcan0 type vcan

$ ip link set up vcan0

$ tc qdisc add dev vcan0 root handle 1: htb

$ tc filter add dev vcan0 parent 1: protocol ip prio 10 \

matchall action skbmod swap mac

Doing the above silently corrupts all the packets. Do not perform skbmod

actions for non-Ethernet packets.