netlink: Don't signify error code

The netlink layer reports negative error code so no need to
signify it.

[xemul: This is what we do for the rest of the nlk code.]

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>