criu/namespaces.c: fix pr_perror usage

1. No newline (pr_perror() already adds one for you)

2. No colon before file name, e.g.

WAS:	Can't open file: /some/file: permission denied
NOW:	Can't open file /some/file: permission denied

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