net: Allow for more then a single subclass for netif_addr_lock

Currently netif_addr_lock_nested assumes that there can be only
a single nesting level between 2 devices.  However, if we
have multiple devices of the same type stacked, this fails.
For example:
 eth0 <-- vlan0.10 <-- vlan0.10.20

A more complicated configuration may stack more then one type of
device in different order.
Ex:
  eth0 <-- vlan0.10 <-- macvlan0 <-- vlan1.10.20 <-- macvlan1

This patch adds an ndo_* function that allows each stackable
device to report its nesting level.  If the device doesn't
provide this function default subclass of 1 is used.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>