net: move secpath_exist helper to sk_buff.h

Future patch will remove skb->sp pointer.
To reduce noise in those patches, move existing helper to
sk_buff and use it in more places to ease skb->sp replacement later.

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