Add taskq_wait_outstanding() function

SPL commit behlendorf/spl@9cef1b5 adds the taskq_wait_outstanding()
interface.  See the commit log for the full justification for this
addition.  This patch adds the required user space counterpart.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>

lib/libzpool/taskq.c

@@ -220,6 +220,12 @@ taskq_wait_id(taskq_t *tq, taskqid_t id)
 	taskq_wait(tq);
 }
 
+void
+taskq_wait_outstanding(taskq_t *tq, taskqid_t id)
+{
+	taskq_wait(tq);
+}
+
 static void
 taskq_thread(void *arg)
 {