files: predump_one_fd -- Use direct read of pid's fd link

Using O_PATH known to be buggy on 3.11 kernel so use
direct link reading procedure here.

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