[IPSEC]: Added xfrm_decode_session_reverse and xfrmX_policy_check_reverse

RFC 4301 requires us to relookup ICMP traffic that does not match any
policies using the reverse of its payload.  This patch adds the functions
xfrm_decode_session_reverse and xfrmX_policy_check_reverse so we can get
the reverse flow to perform such a lookup.

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

include/linux/xfrm.h

@@ -114,6 +114,7 @@ enum
 	XFRM_POLICY_IN	= 0,
 	XFRM_POLICY_OUT	= 1,
 	XFRM_POLICY_FWD	= 2,
+	XFRM_POLICY_MASK = 3,
 	XFRM_POLICY_MAX	= 3
 };