rps: Inspect PPTP encapsulated by GRE to get flow hash

The PPTP is encapsulated by GRE header with that GRE_VERSION bits
must contain one. But current GRE RPS needs the GRE_VERSION must be
zero. So RPS does not work for PPTP traffic.

In my test environment, there are four MIPS cores, and all traffic
are passed through by PPTP. As a result, only one core is 100% busy
while other three cores are very idle. After this patch, the usage
of four cores are balanced well.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Reviewed-by: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/uapi/linux/if_tunnel.h b/include/uapi/linux/if_tunnel.h
index 1046f55..60dbb20 100644
--- a/include/uapi/linux/if_tunnel.h
+++ b/include/uapi/linux/if_tunnel.h
@@ -24,9 +24,14 @@
 #define GRE_SEQ		__cpu_to_be16(0x1000)
 #define GRE_STRICT	__cpu_to_be16(0x0800)
 #define GRE_REC		__cpu_to_be16(0x0700)
-#define GRE_FLAGS	__cpu_to_be16(0x00F8)
+#define GRE_ACK		__cpu_to_be16(0x0080)
+#define GRE_FLAGS	__cpu_to_be16(0x0078)
 #define GRE_VERSION	__cpu_to_be16(0x0007)
 
+#define GRE_VERSION_1	__cpu_to_be16(0x0001)
+#define GRE_PROTO_PPP	__cpu_to_be16(0x880b)
+#define GRE_PPTP_KEY_MASK	__cpu_to_be32(0xffff)
+
 struct ip_tunnel_parm {
 	char			name[IFNAMSIZ];
 	int			link;