net: Zero terminate ifr_name in dev_ifname().

[ Upstream commit 63679112c536289826fec61c917621de95ba2ade ]

The ifr.ifr_name is passed around and assumed to be NULL terminated.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>