Creds are per-thread data, declare them appropriately.
We will need this data to restore threads with different
credentials.

(In a scope of https://jira.sw.ru/browse/PSBM-41416)
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>