Return number of pages freed in the __arc_shrunker_func().

According to the comments in the kernel's shrinker.h file, the
scan_objects callback should return the number of objects actually
freed.  Previously, the shrinker returned the number of objects which
were _attempted_ to be freed.  When the return value of a shrinker is
too high, callers may retry shrinking indefinitely.

module/zfs/arc.c

@@ -2607,7 +2607,7 @@ arc_evictable_memory(void) {
 static int
 __arc_shrinker_func(struct shrinker *shrink, struct shrink_control *sc)
 {
-	uint64_t pages;
+	uint64_t oldpages, pages;
 
 	/* The arc is considered warm once reclaim has occurred */
 	if (unlikely(arc_warm == B_FALSE))
@@ -2632,8 +2632,13 @@ __arc_shrinker_func(struct shrinker *shrink, struct shrink_control *sc)
 	 * reap whatever we can from the various arc slabs.
 	 */
 	if (pages > 0) {
+		oldpages = btop(arc_evictable_memory());
 		arc_kmem_reap_now(ARC_RECLAIM_AGGR, ptob(sc->nr_to_scan));
 		pages = btop(arc_evictable_memory());
+		if (pages > oldpages)
+			pages = pages - oldpages;
+		else
+			pages = 0;
 	} else {
 		arc_kmem_reap_now(ARC_RECLAIM_CONS, ptob(sc->nr_to_scan));
 		pages = -1;