Reduce number of metaslab preload taskq threads.

Before this change ZFS created threads for 50% of CPUs for each top-
level vdev.  Plus it created the same number of threads for embedded
log groups (that have only one metaslab and don't need any preload).
As result, on system with 80 CPUs and pool of 60 vdevs this resulted
in 4800 metaslab preload threads, that is absolutely insane.

This patch changes the preload threads to 50% of CPUs in one taskq
per pool, so on the mentioned system it will be only 40 threads.

Among other things this fixes zdb on the mentioned system and pool
on FreeBSD, that failed to create so many threads in one process.

Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15319

include/sys/metaslab_impl.h

@@ -250,7 +250,6 @@ struct metaslab_group {
 	int64_t			mg_activation_count;
 	metaslab_class_t	*mg_class;
 	vdev_t			*mg_vd;
-	taskq_t			*mg_taskq;
 	metaslab_group_t	*mg_prev;
 	metaslab_group_t	*mg_next;
 

include/sys/spa_impl.h

@@ -424,7 +424,9 @@ struct spa {
 
 	hrtime_t	spa_ccw_fail_time;	/* Conf cache write fail time */
 	taskq_t		*spa_zvol_taskq;	/* Taskq for minor management */
+	taskq_t		*spa_metaslab_taskq;	/* Taskq for metaslab preload */
 	taskq_t		*spa_prefetch_taskq;	/* Taskq for prefetch threads */
+	taskq_t		*spa_upgrade_taskq;	/* Taskq for upgrade jobs */
 	uint64_t	spa_multihost;		/* multihost aware (mmp) */
 	mmp_thread_t	spa_mmp;		/* multihost mmp thread */
 	list_t		spa_leaf_list;		/* list of leaf vdevs */
@@ -448,8 +450,6 @@ struct spa {
 	 */
 	spa_config_lock_t spa_config_lock[SCL_LOCKS]; /* config changes */
 	zfs_refcount_t	spa_refcount;		/* number of opens */
-
-	taskq_t		*spa_upgrade_taskq;	/* taskq for upgrade jobs */
 };
 
 extern char *spa_config_path;