Make error codes returned being a negative value

We have a mess in our return codes:
some functions return negative values,
while others -- positive.

Sanitize the return codes and make error
condition to return negative values.

Reported-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>