nilfs2: use checkpoint tree for mount check of snapshots

This rewrites nilfs_checkpoint_is_mounted() function so that it
decides whether a checkpoint is mounted by whether the corresponding
root object is found in checkpoint tree.
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/the_nilfs.c

@@ -954,26 +954,20 @@ struct nilfs_sb_info *nilfs_find_sbinfo(struct the_nilfs *nilfs,
 int nilfs_checkpoint_is_mounted(struct the_nilfs *nilfs, __u64 cno,
 				int snapshot_mount)
 {
-	struct nilfs_sb_info *sbi;
-	int ret = 0;
+	struct nilfs_root *root;
+	int ret;
 
-	down_read(&nilfs->ns_super_sem);
-	if (cno == 0 || cno > nilfs->ns_cno)
-		goto out_unlock;
+	if (cno < 0 || cno > nilfs->ns_cno)
+		return false;
 
-	list_for_each_entry(sbi, &nilfs->ns_supers, s_list) {
-		if (sbi->s_snapshot_cno == cno &&
-		    (!snapshot_mount || nilfs_test_opt(sbi, SNAPSHOT))) {
-					/* exclude read-only mounts */
-			ret++;
-			break;
-		}
-	}
-	/* for protecting recent checkpoints */
 	if (cno >= nilfs_last_cno(nilfs))
-		ret++;
+		return true;	/* protect recent checkpoints */
 
- out_unlock:
-	up_read(&nilfs->ns_super_sem);
+	ret = false;
+	root = nilfs_lookup_root(nilfs, cno);
+	if (root) {
+		ret = true;
+		nilfs_put_root(root);
+	}
 	return ret;
 }