staging: octeon: fix build failure with XFRM enabled

skb->sp doesn't exist anymore in the next-next tree, so mips defconfig
no longer builds.  Use helper instead to reset the secpath.

Not even compile tested.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 4165079b ("net: switch secpath to use skb extension infrastructure")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>