net: remove ifalias on empty given alias 

This patch removes the potentially allocated ifalias when the (new) given alias is empty.

E.g. when setting

echo "" > /sys/class/net/eth0/ifalias

Signed-off-by: Oliver Hartkopp <oliver@hartkopp.net>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/dev.c b/net/core/dev.c
index e913905..a90737f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -967,6 +967,14 @@
 	if (len >= IFALIASZ)
 		return -EINVAL;
 
+	if (!len) {
+		if (dev->ifalias) {
+			kfree(dev->ifalias);
+			dev->ifalias = NULL;
+		}
+		return 0;
+	}
+
 	dev->ifalias = krealloc(dev->ifalias, len+1, GFP_KERNEL);
 	if (!dev->ifalias)
 		return -ENOMEM;