cr-dump: call stat for /proc/pid/fd/X directly

This patch can be committed instead of:
[PATCH 1/6] cr-dump: move parasite_drain_fds_seized out of dump_task_files
[PATCH 2/6] cr-dump: fix dumping file locks in a mount namespace

readlink is not required here and a file can be unavailable,
if a process is in another mnt namespace

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>