netfilter: ctnetlink: add const qualifier to nfnl_hook.get_ct

get_ct as is and will not update its skb argument, and users of
nfnl_ct_hook is currently only nfqueue, we can add const qualifier.
Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>

include/linux/netfilter.h

@@ -378,7 +378,7 @@ enum ip_conntrack_info;
 struct nlattr;
 
 struct nfnl_ct_hook {
-	struct nf_conn *(*get_ct)(struct sk_buff *skb,
+	struct nf_conn *(*get_ct)(const struct sk_buff *skb,
 				  enum ip_conntrack_info *ctinfo);
 	size_t (*build_size)(const struct nf_conn *ct);
 	int (*build)(struct sk_buff *skb, struct nf_conn *ct,

net/netfilter/nf_conntrack_netlink.c

@@ -2162,7 +2162,7 @@ ctnetlink_glue_build_size(const struct nf_conn *ct)
 	       ;
 }
 
-static struct nf_conn *ctnetlink_glue_get_ct(struct sk_buff *skb,
+static struct nf_conn *ctnetlink_glue_get_ct(const struct sk_buff *skb,
 					     enum ip_conntrack_info *ctinfo)
 {
 	struct nf_conn *ct;