rst: Fix creds vs threads restoration

Writing to last_pid sysctl is CAP_SYS_ADMIN potected. Thus restoring
creds before it won't work in all the cases.

Fix this by making all threads restore creds themselves, and the
thread group leader -- after all of them.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrey Vagin <avagin@parallels.com>