parse_pid_stat(): use pr_err()

We do not want to use pr_perror() here, because

1. fscanf only sets errno in case it returns EOF
2. we are not really interested in errno value

So use pr_err() instead.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>