mmc: sdhci: Setting the host->mrq to NULL before executing tuning

In function sdhci_request(), it is possible to do the tuning execution
like below:

sdhci_request() {
	spin_lock_irqsave(&host->lock, flags);
	host->mrq = mrq;
	...
	spin_unlock_irqrestore(&host->lock, flags);

<=== Here it is possible one pending finish_tasklet get running
     and it will operate the original mrq, and notified the mrq
     is done, and causes memory corruption.

	sdhci_execute_tuning(mmc, tuning_opcode);
	spin_lock_irqsave(&host->lock, flags);
	host->mrq = mrq;
...
}

In the above race place, the original mrq should not be finished wrongly,
so here before unlock the spinlock, we need to set the host->mrq to NULL
to avoid this case.
Signed-off-by: Liu, Chuansheng <chuansheng.liu@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/host/sdhci.c

@@ -1396,6 +1396,13 @@ static void sdhci_request(struct mmc_host *mmc, struct mmc_request *mrq)
 					mmc->card->type == MMC_TYPE_MMC ?
 					MMC_SEND_TUNING_BLOCK_HS200 :
 					MMC_SEND_TUNING_BLOCK;
+
+				/* Here we need to set the host->mrq to NULL,
+				 * in case the pending finish_tasklet
+				 * finishes it incorrectly.
+				 */
+				host->mrq = NULL;
+
 				spin_unlock_irqrestore(&host->lock, flags);
 				sdhci_execute_tuning(mmc, tuning_opcode);
 				spin_lock_irqsave(&host->lock, flags);