Use skb_sec_path and secpath_exists helpers where possible.
This reduces noise in followup patch that removes skb->sp pointer.

v2: no changes, preseve acks from v1.
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>