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/zio.c

@@ -1557,7 +1557,7 @@ zio_delay_interrupt(zio_t *zio)
 				tid = taskq_dispatch_delay(system_taskq,
 				    (task_func_t *) zio_interrupt,
 				    zio, TQ_NOSLEEP, expire_at_tick);
-				if (!tid) {
+				if (tid == TASKQID_INVALID) {
 					/*
 					 * Couldn't allocate a task.  Just
 					 * finish the zio without a delay.