net: use skb_sec_path helper in more places

skb_sec_path gains 'const' qualifier to avoid
xt_policy.c: 'skb_sec_path' discards 'const' qualifier from pointer target type

same reasoning as previous conversions: Won't need to touch these
spots anymore when skb->sp is removed.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>