It's useful to know when the dump occurred, so add a timestamp file for
this.

Link: https://lore.kernel.org/r/1571926105-74636-6-git-send-email-john.garry@huawei.comSigned-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>