amt: do not use overwrapped cb area

amt driver uses skb->cb for storing tunnel information.
This job is worked before TC layer and then amt driver load tunnel info
from skb->cb after TC layer.
So, its cb area should not be overwrapped with CB area used by TC.
In order to not use cb area used by TC, it skips the biggest cb
structure used by TC, which was qdisc_skb_cb.
But it's not anymore.
Currently, biggest structure of TC's CB is tc_skb_cb.
So, it should skip size of tc_skb_cb instead of qdisc_skb_cb.

Fixes: ec624fe7

 ("net/sched: Extend qdisc control block with tc control block")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20240107144241.4169520-1-ap420073@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>