genetlink: constify genl_err_attr() argument

genl_err_attr() sets netlink_ext_ack::bad_attr which is a pointer to const
struct nlattr so make the attr argument also const.
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/genetlink.h

@@ -112,7 +112,7 @@ static inline void genl_info_net_set(struct genl_info *info, struct net *net)
 #define GENL_SET_ERR_MSG(info, msg) NL_SET_ERR_MSG((info)->extack, msg)
 
 static inline int genl_err_attr(struct genl_info *info, int err,
-				struct nlattr *attr)
+				const struct nlattr *attr)
 {
 	info->extack->bad_attr = attr;