netlink: constify nlmsghdr arguments

Consitfy nlmsghdr arguments to a couple of functions as preparation
for the next patch, which will constify the netlink message data in
all nfnetlink users.

Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 5ba398e..0fbecbb 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -217,12 +217,13 @@
 
 struct netlink_callback
 {
-	struct sk_buff	*skb;
-	struct nlmsghdr	*nlh;
-	int		(*dump)(struct sk_buff * skb, struct netlink_callback *cb);
-	int		(*done)(struct netlink_callback *cb);
-	int		family;
-	long		args[6];
+	struct sk_buff		*skb;
+	const struct nlmsghdr	*nlh;
+	int			(*dump)(struct sk_buff * skb,
+					struct netlink_callback *cb);
+	int			(*done)(struct netlink_callback *cb);
+	int			family;
+	long			args[6];
 };
 
 struct netlink_notify
@@ -258,7 +259,7 @@
 	NLMSG_NEW(skb, pid, seq, type, len, 0)
 
 extern int netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
-			      struct nlmsghdr *nlh,
+			      const struct nlmsghdr *nlh,
 			      int (*dump)(struct sk_buff *skb, struct netlink_callback*),
 			      int (*done)(struct netlink_callback*));