[XFRM]: Prevent off-by-one access to xfrm_dispatch

Makes the type > XFRM_MSG_MAX check behave correctly to
protect access to xfrm_dispatch.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/xfrm.h

@@ -140,8 +140,9 @@ enum {
 	XFRM_MSG_FLUSHPOLICY,
 #define XFRM_MSG_FLUSHPOLICY XFRM_MSG_FLUSHPOLICY
 
-	XFRM_MSG_MAX
+	__XFRM_MSG_MAX
 };
+#define XFRM_MSG_MAX (__XFRM_MSG_MAX - 1)
 
 struct xfrm_user_tmpl {
 	struct xfrm_id		id;