files-reg: Use pr_err instead of pr_perror if fd not found

Otherwise

 | (00.002843)      1: Error (files-reg.c:522): Can't find regfile for 0
 | : Success

which is inappropriate here.

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