[IPSEC]: Fix reversed ICMP6 policy check

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: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>