rtnetlink: remove check for fill_slave_info in rtnl_have_link_slave_info

This check is not needed because the same check is done before
fill_slave_info is used in rtnl_link_slave_info_fill.
Also, by removing this check, kernel will fillup IFLA_INFO_SLAVE_KIND
even for slaves of masters which does not implement fill_slave_info.

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