dmaengine: idxd: add RO check for wq max_batch_size write

Block wq_max_batch_size_store() when the device is configured as read-only
and not configurable.

Fixes: e7184b159d ("dmaengine: idxd: add support for configurable max wq batch size")
Reported-by: Bernice Zhang <bernice.zhang@intel.com>
Tested-by: Bernice Zhang <bernice.zhang@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/164971493551.2201159.1942042593642155209.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/idxd/sysfs.c

@@ -939,6 +939,9 @@ static ssize_t wq_max_batch_size_store(struct device *dev, struct device_attribu
 	u64 batch_size;
 	int rc;
 
+	if (!test_bit(IDXD_FLAG_CONFIGURABLE, &idxd->flags))
+		return -EPERM;
+
 	if (wq->state != IDXD_WQ_DISABLED)
 		return -EPERM;