fix dnode eviction typo in arc_evict_state()

In addition decrease arc_prune_taskq max thread count to avoid
consuming all the cores when the system is under memory pressure and
is trying to reclaim memory from the dentry/inode caches.
closes openzfs#7559

Signed-off-by: Alek Pinchuk <apinchuk@axcient.com>

module/zfs/arc.c

@@ -4043,14 +4043,17 @@ arc_evict_state(arc_state_t *state, uint64_t spa, int64_t bytes,
 
 		/*
 		 * Try to reduce pinned dnodes with a floor of arc_dnode_limit.
-		 * Request that 10% of the LRUs be scanned by the superblock
-		 * shrinker.
+		 * Request that zfs_arc_dnode_reduce_percent of the LRUs be
+		 * scanned by the superblock shrinker.
 		 */
-		if (type == ARC_BUFC_DATA && aggsum_compare(&astat_dnode_size,
+		if (aggsum_compare(&astat_dnode_size,
 		    arc_dnode_size_limit) > 0) {
+			ASSERT(type == ARC_BUFC_DATA ||
+			    type == ARC_BUFC_METADATA);
 			arc_prune_async((aggsum_upper_bound(&astat_dnode_size) -
 			    arc_dnode_size_limit) / sizeof (dnode_t) /
 			    zfs_arc_dnode_reduce_percent);
+			taskq_wait(arc_prune_taskq);
 		}
 
 		/*
@@ -7348,8 +7351,7 @@ arc_init(void)
 	    offsetof(arc_prune_t, p_node));
 	mutex_init(&arc_prune_mtx, NULL, MUTEX_DEFAULT, NULL);
 
-	arc_prune_taskq = taskq_create("arc_prune", boot_ncpus, defclsyspri,
-	    boot_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
+	arc_prune_taskq = taskq_create("arc_prune", 1, defclsyspri, 0, 0, 0);
 
 	arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
 	    sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);