Return number of object freed in the generic shrinker

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.  It also didn't cap the freeing
at nr_to_scan.  If the value returned is too high, callers may retry
shrinking indefinitely.

module/spl/spl-kmem.c

@@ -64,7 +64,7 @@ MODULE_PARM_DESC(spl_kmem_cache_expire, "By age (0x1) or low memory (0x2)");
  * setting this value to KMC_RECLAIM_ONCE limits how aggressively the cache
  * is reclaimed.  This may increase the likelihood of out of memory events.
  */
-unsigned int spl_kmem_cache_reclaim = 0;
+unsigned int spl_kmem_cache_reclaim = 0 /* KMC_RECLAIM_ONCE */;
 module_param(spl_kmem_cache_reclaim, uint, 0644);
 MODULE_PARM_DESC(spl_kmem_cache_reclaim, "Single reclaim pass (0x1)");
 
@@ -2249,21 +2249,22 @@ __spl_kmem_cache_generic_shrinker(struct shrinker *shrink,
     struct shrink_control *sc)
 {
 	spl_kmem_cache_t *skc;
+	uint64_t oldalloc;
 	int alloc = 0;
 
 	down_read(&spl_kmem_cache_sem);
 	list_for_each_entry(skc, &spl_kmem_cache_list, skc_list) {
-		if (sc->nr_to_scan)
+		if (sc->nr_to_scan) {
+			oldalloc = skc->skc_obj_alloc;
 			spl_kmem_cache_reap_now(skc,
 			   MAX(sc->nr_to_scan >> fls64(skc->skc_slab_objs), 1));
-
-		/*
-		 * Presume everything alloc'ed is reclaimable, this ensures
-		 * we are called again with nr_to_scan > 0 so can try and
-		 * reclaim.  The exact number is not important either so
-		 * we forgo taking this already highly contented lock.
-		 */
-		alloc += skc->skc_obj_alloc;
+			if (oldalloc > skc->skc_obj_alloc)
+				alloc += oldalloc - skc->skc_obj_alloc;
+		} else {
+			alloc += skc->skc_obj_alloc;
+		}
+		if (alloc >= sc->nr_to_scan)
+			break;
 	}
 	up_read(&spl_kmem_cache_sem);
 
@@ -2273,7 +2274,7 @@ __spl_kmem_cache_generic_shrinker(struct shrinker *shrink,
 	 * shrink_slabs() is repeatedly invoked by many cores causing the
 	 * system to thrash.
 	 */
-	if ((spl_kmem_cache_reclaim & KMC_RECLAIM_ONCE) && sc->nr_to_scan)
+	if ((spl_kmem_cache_reclaim & KMC_RECLAIM_ONCE))
 		return (-1);
 
 	return MAX((alloc * sysctl_vfs_cache_pressure) / 100, 0);