net: sched: fix TCF_LAYER_LINK case in tcf_get_base_ptr

TCF_LAYER_LINK and TCF_LAYER_NETWORK returned the same pointer as
skb->data points to the network header.
Use skb_mac_header instead.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>