Avoid some spa_has_pending_synctask() calls.

Since 8c4fb36 (PR openzfs#7795) spa_has_pending_synctask() started to
take two more locks per write inside txg_all_lists_empty().  I am
surprised those pool-wide locks are not contended, but still their
operations are visible in CPU profiles under contended vdev lock.

This commit slightly changes vdev_queue_max_async_writes() flow to
not call the function if we are going to return max_active any way
due to high amount of dirty data.  It allows to save some CPU time
exactly when the pool is busy.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-By: Tom Caputi <caputit1@tcnj.edu>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes openzfs#11280

module/zfs/vdev_queue.c

@@ -338,14 +338,12 @@ vdev_queue_max_async_writes(spa_t *spa)
 	 * Sync tasks correspond to interactive user actions. To reduce the
 	 * execution time of those actions we push data out as fast as possible.
 	 */
-	if (spa_has_pending_synctask(spa))
+	dirty = dp->dp_dirty_total;
+	if (dirty > max_bytes || spa_has_pending_synctask(spa))
 		return (zfs_vdev_async_write_max_active);
 
-	dirty = dp->dp_dirty_total;
 	if (dirty < min_bytes)
 		return (zfs_vdev_async_write_min_active);
-	if (dirty > max_bytes)
-		return (zfs_vdev_async_write_max_active);
 
 	/*
 	 * linear interpolation: