dump: Don't shadow the ret code with dump_namespaces

cr_dump_tasks does assign ret = -1 by default but
dump_namespaces does shadow this variable with own
error code so that if any subsequent calls will fail
we will not notice the error.

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