Add assert under lock to detect cases of dispach of a preallocated

taskq work item to more than one queue concurrently. Also, please
see discussion in openzfs#3840.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Boris Protopopov <boris.protopopov@actifio.com>
Closes openzfs#609

module/spl/spl-taskq.c

@@ -683,6 +683,12 @@ taskq_dispatch_ent(taskq_t *tq, task_func_t func, void *arg, uint_t flags,
 
 	spin_lock(&t->tqent_lock);
 
+	/*
+	 * Make sure the entry is not on some other taskq; it is important to
+	 * ASSERT() under lock
+	 */
+	ASSERT(taskq_empty_ent(t));
+
 	/*
 	 * Mark it as a prealloc'd task.  This is important
 	 * to ensure that we don't free it later.