Fix accounting error for pending sync IO ops in zpool iostat

Currently vdev_queue_class_length is responsible for checking how long
the queue length is, however, it doesn't check the length when a list
is used, rather it just returns whether it is empty or not. To fix this
I added a counter variable to vdev_queue_class to keep track of the sync
IO ops, and changed vdev_queue_class_length to reference this variable
instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: MigeljanImeri <ImeriMigel@gmail.com>
Closes #15478

include/sys/vdev_impl.h

@@ -131,7 +131,10 @@ typedef const struct vdev_ops {
  * Virtual device properties
  */
 typedef union vdev_queue_class {
-	list_t		vqc_list;
+	struct {
+		ulong_t 	vqc_list_numnodes;
+		list_t		vqc_list;
+	};
 	avl_tree_t	vqc_tree;
 } vdev_queue_class_t;