The policy check I added for ICMP on IPv6 is reversed.  This
patch fixes that.

It also adds an skb->sp check so that unprotected packets that
fail the policy check do not crash the machine.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>