[NET] netconsole: Introduce netconsole_netdev_notifier

Based upon initial work by Keiichi Kii <k-keiichi@bx.jp.nec.com>.

To update fields of underlying netpoll structure at runtime on corresponding
NETDEV_CHANGEADDR or NETDEV_CHANGENAME notifications.

ioctl(SIOCSIFHWADDR or SIOCSIFNAME) could be used to change the hardware/MAC
address or name of the local interface that our netpoll is attached to.
Whenever this happens, netdev notifier chain is called out with the
NETDEV_CHANGEADDR or NETDEV_CHANGENAME event message.  We respond to that and
update the local_mac or dev_name field of the struct netpoll.  This makes
sense anyway, but is especially required for dynamic netconsole because the
netpoll structure's internal members become user visible files when either
sysfs or configfs are used.  So this helps us to keep up with the MAC
address/name changes and keep values in struct netpoll uptodate.

[ Note that ioctl(SIOCSIFADDR) to change IP address of interface at
  runtime is not handled (to update local_ip of netpoll) on purpose --
  some setups may set the local_ip to a private address, not necessary
  the actual IP address of the sender host, as presently allowed. ]

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Signed-off-by: Keiichi Kii <k-keiichi@bx.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>