pie: open_detach_mount -- Use pr_perror for regular build

This helper is used in both PIE and non-PIE code.
For the second we can use normal pr_perror.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>