zdtm unix_sock: don't use errno

In this place, errno is not being set and can be 0 or some random value,
so we should not use it. In case errno happens to be 0, we end up returning
a success status from a function while we should have returned a failure.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrew Vagin <avagin@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>