Add new rtnl_link_af_data_compare function to compare af_data

This patch adds a new api rtnl_linl_af_data_compare to
compare link af_data

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
link: Add af data compare to link objects

Today the link compare function does not
compare af data of a link. We have found a
need for this to get approriate change callbacks
when af_data of a link changes.

This patch adds support to compare af_data
to link_compare function. This patch today
only adds support to compare af_data set by
PROTINFO attributes. It can be extended to
support compares of af_data set by AF_SPEC
attributes

It has been tested for AF_BRIDGE objects.
In case of AF_BRIDGE objects, this helps with
bridge port change notification callbacks.

Signed-off-by: Wilson Kok <wkok@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
link: fix reference leak in rtnl_link_af_data_compare()

Signed-off-by: Thomas Graf <tgraf@suug.ch>