Reduce number of arc_prune threads.

On FreeBSD vnode reclamation is single-threaded, protected by single
global lock.  Having more threads there is just a waste of resources,
so reduce it to one.

Linux uses 1-2 threads per mount point, but even if it can scale
that far, I doubt it makes sense to use all CPUs for it same time,
so limit it to 80% alike to batch ZIO taskqs.

Signed-off-by: Alexander Motin <mav@FreeBSD.org>

module/zfs/arc.c

@@ -7983,7 +7983,12 @@ 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", 100, defclsyspri,
+#ifdef __FreeBSD__
+	percent = 0;	/* More than one thread makes no sense. */
+#else
+	percent = 80;	/* Can use 1-2 threads per mount point. */
+#endif
+	arc_prune_taskq = taskq_create("arc_prune", percent, defclsyspri,
 	    boot_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC |
 	    TASKQ_THREADS_CPU_PCT);