netconsole: enable netconsole can make net_device refcnt incorrent

There is no check if netconsole is enabled current.
so when exec echo 1 > enabled;
the reference of net_device will increment always.

Signed-off-by: Gao feng <[e-mail]>
Acked-by: Flavio Leitner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/netconsole.c

···
 		return err;
 	if (enabled < 0 || enabled > 1)
 		return -EINVAL;
+	if (enabled == nt->enabled) {
+		printk(KERN_INFO "netconsole: network logging has already %s\n",
+				nt->enabled ? "started" : "stopped");
+		return -EINVAL;
+	}
 
 	if (enabled) {	/* 1 */