netconsole: avoid null pointer dereference at show_local_mac()

This patch avoids a null pointer dereference when we read local_mac
for netconsole in configfs and shows default local mac address
value.

A null pointer dereference occurs when we call show_local_mac() via
local_mac entry in configfs before we setup the content of netpoll
using netpoll_setup().

Signed-off-by: Keiichi KII <k-keiichi@bx.jp.nec.com>
Signed-off-by: David S. Miller <davem@davemloft.net>