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/arc.c

@@ -3479,7 +3479,7 @@ arc_prune_async(int64_t adjust)
 		refcount_add(&ap->p_refcnt, ap->p_pfunc);
 		ap->p_adjust = adjust;
 		if (taskq_dispatch(arc_prune_taskq, arc_prune_task,
-		    ap, TQ_SLEEP) == 0) {
+		    ap, TQ_SLEEP) == TASKQID_INVALID) {
 			refcount_remove(&ap->p_refcnt, ap->p_pfunc);
 			continue;
 		}