[SCSI] uas: disable use of blk-mq I/O path

The uas driver uses the block layer tag for USB3 stream IDs.  With
blk-mq we can get larger tag numbers that the queue depth, which breaks
this assumption.  A fix is under way for 3.18, but sits on top of
large changes so can't easily be backported.   Set the disable_blk_mq
path so that a uas device can't easily crash the system when using
blk-mq for SCSI.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/usb/storage/uas.c

@@ -970,6 +970,13 @@ static struct scsi_host_template uas_host_template = {
 	.cmd_per_lun = 1,	/* until we override it */
 	.skip_settle_delay = 1,
 	.ordered_tag = 1,
+
+	/*
+	 * The uas drivers expects tags not to be bigger than the maximum
+	 * per-device queue depth, which is not true with the blk-mq tag
+	 * allocator.
+	 */
+	.disable_blk_mq = true,
 };
 
 #define UNUSUAL_DEV(id_vendor, id_product, bcdDeviceMin, bcdDeviceMax, \