nlmon: use standard rtnetlink link api for add/del devices

It is not nice when netdev is created right after module load and with
some implicit name. So rather change nlmon to use standard rtnl link API.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>