net: tipc: allocate attrs locally instead of using genl_family_attrbuf in compat_dumpit()

As this is the last user of genl_family_attrbuf, convert to allocate
attrs locally and do it in a similar way this is done in compat_doit().

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