criu/namespaces.c: correct pr_err vs pr_perror usage

So, in places where we have errno available (and it makes sense
to show it), we want to use pr_perror().

In places where errno is not set/used, use pr_err().

Cc: Dengguangxing <dengguangxing@huawei.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>