datapath: test for netlink_set_err returning void

datapath: test for netlink_set_err returning void

In v2.6.33 netlink_set_err returns void. However, 1a50307ba182 ("netlink:
fix NETLINK_RECV_NO_ENOBUFS in netlink_set_err()") was backported and
included in v2.6.33.2 and in that and subsequent v2.6.33 stable releases
netlink_set_err returns an int.

It seems plausible that there are other backports floating around. So check
for netlink_set_err returning void rather than including compatibility code
based on the version of the kernel.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>