scsi: fnic: fix use after free

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>

drivers/scsi/fnic/fnic_scsi.c

@@ -1024,7 +1024,8 @@ static void fnic_fcpio_icmnd_cmpl_handler(struct fnic *fnic,
 		atomic64_inc(&fnic_stats->io_stats.io_completions);
 
 
-	io_duration_time = jiffies_to_msecs(jiffies) - jiffies_to_msecs(io_req->start_time);
+	io_duration_time = jiffies_to_msecs(jiffies) -
+						jiffies_to_msecs(start_time);
 
 	if(io_duration_time <= 10)
 		atomic64_inc(&fnic_stats->io_stats.io_btw_0_to_10_msec);