nilfs2: get rid of BUG_ON use in btree lookup routines

The current btree lookup routines make a kernel oops when detected
inconsistency in btree blocks.  These routines should instead return a
proper error code because the inconsistency usually comes from
corruption of on-disk metadata.

This fixes the issue by converting BUG_ON calls to proper error
handlings.
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/btree.c

@@ -444,6 +444,18 @@ nilfs_btree_get_node(const struct nilfs_btree *btree,
 		nilfs_btree_get_nonroot_node(path, level);
 }
 
+static inline int
+nilfs_btree_bad_node(struct nilfs_btree_node *node, int level)
+{
+	if (unlikely(nilfs_btree_node_get_level(node) != level)) {
+		dump_stack();
+		printk(KERN_CRIT "NILFS: btree level mismatch: %d != %d\n",
+		       nilfs_btree_node_get_level(node), level);
+		return 1;
+	}
+	return 0;
+}
+
 static int nilfs_btree_do_lookup(const struct nilfs_btree *btree,
 				 struct nilfs_btree_path *path,
 				 __u64 key, __u64 *ptrp, int minlevel)
@@ -467,7 +479,8 @@ static int nilfs_btree_do_lookup(const struct nilfs_btree *btree,
 		if (ret < 0)
 			return ret;
 		node = nilfs_btree_get_nonroot_node(path, level);
-		BUG_ON(level != nilfs_btree_node_get_level(node));
+		if (nilfs_btree_bad_node(node, level))
+			return -EINVAL;
 		if (!found)
 			found = nilfs_btree_node_lookup(node, key, &index);
 		else
@@ -512,7 +525,8 @@ static int nilfs_btree_do_lookup_last(const struct nilfs_btree *btree,
 		if (ret < 0)
 			return ret;
 		node = nilfs_btree_get_nonroot_node(path, level);
-		BUG_ON(level != nilfs_btree_node_get_level(node));
+		if (nilfs_btree_bad_node(node, level))
+			return -EINVAL;
 		index = nilfs_btree_node_get_nchildren(node) - 1;
 		ptr = nilfs_btree_node_get_ptr(btree, node, index);
 		path[level].bp_index = index;