Enable linker versioning

Reported-by: Ian Stakenvicius
Signed-off-by: Thomas Graf <tgraf@suug.ch>
fix double free caused by freeing link af_data in rtnl_link_set_family()

Introduced by commit 8026fe2e3a9089eff3f5a06ee6e3cc78d96334ed ("link:
Free and realloc af specific data upon rtnl_link_set_family()")

link->l_af_data[link->l_af_ops->ao_family] is freed here but not set to
zero. That leads to double free made by link_free_data->do_foreach_af.

Fix this by setting link->l_af_data[link->l_af_ops->ao_family] to zero
rigth after free.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Thomas Graf <tgraf@suug.ch>