[NETFILTER]: nf_{conntrack,nat}_proto_gre: annotate with const

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netfilter/nf_conntrack_proto_gre.c b/net/netfilter/nf_conntrack_proto_gre.c
index 4a185f6..e10024a 100644
--- a/net/netfilter/nf_conntrack_proto_gre.c
+++ b/net/netfilter/nf_conntrack_proto_gre.c
@@ -161,9 +161,11 @@
 			   unsigned int dataoff,
 			   struct nf_conntrack_tuple *tuple)
 {
-	struct gre_hdr_pptp _pgrehdr, *pgrehdr;
+	const struct gre_hdr_pptp *pgrehdr;
+	struct gre_hdr_pptp _pgrehdr;
 	__be16 srckey;
-	struct gre_hdr _grehdr, *grehdr;
+	const struct gre_hdr *grehdr;
+	struct gre_hdr _grehdr;
 
 	/* first only delinearize old RFC1701 GRE header */
 	grehdr = skb_header_pointer(skb, dataoff, sizeof(_grehdr), &_grehdr);