mmc: block: Let MMC_IOC_MULTI_CMD return zero again for zero entries

With gcc 4.1.2:

    drivers/mmc/core/block.c: In function ‘mmc_blk_ioctl_cmd_issue’:
    drivers/mmc/core/block.c:630: warning: ‘ioc_err’ may be used uninitialized in this function

Indeed, if mq_rq->ioc_count is zero, an uninitialized value will be
stored in mq_rq->drv_op_result and passed to blk_end_request_all().

Can mq_rq->ioc_count be zero?
  - mmc_blk_ioctl_cmd() sets ioc_count to 1, so this is safe,
  - mmc_blk_ioctl_multi_cmd() obtains ioc_count from user space in
    response to the MMC_IOC_MULTI_CMD ioctl, and does allow zero.

To avoid returning an uninitialized value, and as it is pointless to do
all this work when the MMC_IOC_MULTI_CMD ioctl is used with zero
entries, check for this early in mmc_blk_ioctl_multi_cmd(), and return
zero, like was returned before.

Fixes: 3ecd8cf23f ("mmc: block: move multi-ioctl() to use block layer")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/block.c

@@ -637,6 +637,9 @@ static int mmc_blk_ioctl_multi_cmd(struct block_device *bdev,
 			   sizeof(num_of_cmds)))
 		return -EFAULT;
 
+	if (!num_of_cmds)
+		return 0;
+
 	if (num_of_cmds > MMC_IOC_MAX_CMDS)
 		return -EINVAL;