Return -1 from arc_shrinker_func()

This is analogous to SPL commit openzfs/spl@b9b3715.  While
we don't have clear evidence of systems getting caught here
indefinately like in the SPL this ensures that it will never
happen.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#1579

module/zfs/arc.c

@@ -2374,10 +2374,8 @@ __arc_shrinker_func(struct shrinker *shrink, struct shrink_control *sc)
 	 */
 	if (pages > 0) {
 		arc_kmem_reap_now(ARC_RECLAIM_AGGR, ptob(sc->nr_to_scan));
-		pages = btop(arc_evictable_memory());
 	} else {
 		arc_kmem_reap_now(ARC_RECLAIM_CONS, ptob(sc->nr_to_scan));
-		pages = -1;
 	}
 
 	/*
@@ -2397,7 +2395,7 @@ __arc_shrinker_func(struct shrinker *shrink, struct shrink_control *sc)
 
 	mutex_exit(&arc_reclaim_thr_lock);
 
-	return (pages);
+	return (-1);
 }
 SPL_SHRINKER_CALLBACK_WRAPPER(arc_shrinker_func);