f2fs: avoid casted negative value as shrink count

This patch makes sure it returns a positive value instead of a probable
casted negative value as shrink count.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/shrinker.c

@@ -21,14 +21,16 @@ static unsigned int shrinker_run_no;
 
 static unsigned long __count_nat_entries(struct f2fs_sb_info *sbi)
 {
-	return NM_I(sbi)->nat_cnt - NM_I(sbi)->dirty_nat_cnt;
+	long count = NM_I(sbi)->nat_cnt - NM_I(sbi)->dirty_nat_cnt;
+
+	return count > 0 ? count : 0;
 }
 
 static unsigned long __count_free_nids(struct f2fs_sb_info *sbi)
 {
-	if (NM_I(sbi)->nid_cnt[FREE_NID_LIST] > MAX_FREE_NIDS)
-		return NM_I(sbi)->nid_cnt[FREE_NID_LIST] - MAX_FREE_NIDS;
-	return 0;
+	long count = NM_I(sbi)->nid_cnt[FREE_NID_LIST] - MAX_FREE_NIDS;
+
+	return count > 0 ? count : 0;
 }
 
 static unsigned long __count_extent_cache(struct f2fs_sb_info *sbi)