Add TASKQID_INVALID

Add the TASKQID_INVALID macros and update callers to use the macro
instead of testing against 0.  There is no functional change
even though the functions in zfs_ctldir.c incorrectly used -1
instead of 0.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5347

module/zfs/vdev.c

@@ -1191,7 +1191,7 @@ retry_sync:
 
 		for (c = 0; c < children; c++)
 			VERIFY(taskq_dispatch(tq, vdev_open_child,
-			    vd->vdev_child[c], TQ_SLEEP) != 0);
+			    vd->vdev_child[c], TQ_SLEEP) != TASKQID_INVALID);
 
 		taskq_destroy(tq);
 	}