drivers: net: netdevsim: use skb_sec_path helper

... so this won't have to be changed when skb->sp goes away.

v2: no changes, preserve ack.

Acked-by: Shannon Nelson <shannon.lee.nelson@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>