genetlink: make genl_info->nlhdr const

struct netlink_callback has a const nlh pointer, make the
pointer in struct genl_info const as well, to make copying
between the two easier.

Reviewed-by: Jiri Pirko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/genetlink.h

···
 struct genl_info {
 	u32			snd_seq;
 	u32			snd_portid;
-	struct nlmsghdr *	nlhdr;
+	const struct nlmsghdr *	nlhdr;
 	struct genlmsghdr *	genlhdr;
 	void *			userhdr;
 	struct nlattr **	attrs;

net/ncsi/ncsi-netlink.c

···
 int ncsi_send_netlink_err(struct net_device *dev,
 			  u32 snd_seq,
 			  u32 snd_portid,
-			  struct nlmsghdr *nlhdr,
+			  const struct nlmsghdr *nlhdr,
 			  int err)
 {
 	struct nlmsghdr *nlh;

net/ncsi/ncsi-netlink.h

···
 int ncsi_send_netlink_err(struct net_device *dev,
 			  u32 snd_seq,
 			  u32 snd_portid,
-			  struct nlmsghdr *nlhdr,
+			  const struct nlmsghdr *nlhdr,
 			  int err);
 
 #endif /* __NCSI_NETLINK_H__ */