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>