rtnetlink: Add dump all for netconf

Use the rtnl_dump_all to dump all netconf handlers that have been
registered. Allows userspace to send a dump request for PF_UNSPEC
and get all families.

Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>