lib/route: Allow override of IFLA_AF_SPEC nesting

lib/route: Allow override of IFLA_AF_SPEC nesting

This patch adds the ability to override nesting into an AF specific
attribute. An example of this is the bridge module.

Regular Nesting:
[IFLA_AF_SPEC]
    [AF_INET]
        [AF_INET_ATTRS]

Bridge Nesting:
[IFLA_AF_SPEC]
    [AF_BRIDGE_ATTRS]

This patch adds ao_fill_af_no_nest to struct rtnl_link_af_ops.
When set to non-zero, this will override the nested AF attribute
and allow nesting of attributes directly into IFLA_AF_SPEC.

Signed-off-by: Jef Oliver <jef.oliver@intel.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>