f2fs: fix to return SHRINK_EMPTY if no objects to free

Quoted from include/linux/shrinker.h
"count_objects should return the number of freeable items in the cache. If
 there are no objects to free, it should return SHRINK_EMPTY, while 0 is
 returned in cases of the number of freeable items cannot be determined
 or shrinker should skip this cache for this time (e.g., their number
 is below shrinkable limit)."

Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/shrinker.c

@@ -73,7 +73,7 @@ unsigned long f2fs_shrink_count(struct shrinker *shrink,
 		mutex_unlock(&sbi->umount_mutex);
 	}
 	spin_unlock(&f2fs_list_lock);
-	return count;
+	return count ?: SHRINK_EMPTY;
 }
 
 unsigned long f2fs_shrink_scan(struct shrinker *shrink,