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/metaslab.c

@@ -1724,7 +1724,7 @@ metaslab_group_preload(metaslab_group_t *mg)
 		 */
 		mutex_exit(&mg->mg_lock);
 		VERIFY(taskq_dispatch(mg->mg_taskq, metaslab_preload,
-		    msp, TQ_SLEEP) != 0);
+		    msp, TQ_SLEEP) != TASKQID_INVALID);
 		mutex_enter(&mg->mg_lock);
 		msp = msp_next;
 	}