net/sched: act_mirred: Refactor detection whether dev needs xmit at mac header

Move detection logic that tests whether device expects skb data to point
at mac_header upon xmit into a function.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>