net: push loops and nb calls into helper functions

Push iterations over net namespaces and netdevices from
register_netdevice_notifier() and unregister_netdevice_notifier()
into helper functions. Along with that introduce continue_reverse macros
to make the code a bit nicer allowing to get rid of "last" marks.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>