raid5: allow arbitrary max_hw_sectors

raid5 will split bio to proper size internally, there is no point to use
underlayer disk's max_hw_sectors. In my qemu system, without the change,
the raid5 only receives 128k size bio, which reduces the chance of bio
merge sending to underlayer disks.
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid5.c

@@ -7066,6 +7066,8 @@ static int raid5_run(struct mddev *mddev)
 		else
 			queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD,
 						mddev->queue);
+
+		blk_queue_max_hw_sectors(mddev->queue, UINT_MAX);
 	}
 
 	if (journal_dev) {