The memory chunk io_req is released by mempool_free. Accessing
io_req->start_time will result in a use after free bug. The variable
start_time is a backup of the timestamp. So, use start_time here to
avoid use after free.

Link: https://lore.kernel.org/r/1572881182-37664-1-git-send-email-bianpan2016@163.comSigned-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Satish Kharat <satishkh@cisco.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>