btrfs: do proper error handling in create_reloc_root

We do memory allocations here, read blocks from disk, all sorts of
operations that could easily fail at any given point.  Instead of
panicing the box, simply return the error back up the chain, all callers
at this point have proper error handling.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -733,10 +733,12 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
 	struct extent_buffer *eb;
 	struct btrfs_root_item *root_item;
 	struct btrfs_key root_key;
-	int ret;
+	int ret = 0;
+	bool must_abort = false;
 
 	root_item = kmalloc(sizeof(*root_item), GFP_NOFS);
-	BUG_ON(!root_item);
+	if (!root_item)
+		return ERR_PTR(-ENOMEM);
 
 	root_key.objectid = BTRFS_TREE_RELOC_OBJECTID;
 	root_key.type = BTRFS_ROOT_ITEM_KEY;
@@ -748,7 +750,9 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
 		/* called by btrfs_init_reloc_root */
 		ret = btrfs_copy_root(trans, root, root->commit_root, &eb,
 				      BTRFS_TREE_RELOC_OBJECTID);
-		BUG_ON(ret);
+		if (ret)
+			goto fail;
+
 		/*
 		 * Set the last_snapshot field to the generation of the commit
 		 * root - like this ctree.c:btrfs_block_can_be_shared() behaves
@@ -769,9 +773,16 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
 		 */
 		ret = btrfs_copy_root(trans, root, root->node, &eb,
 				      BTRFS_TREE_RELOC_OBJECTID);
-		BUG_ON(ret);
+		if (ret)
+			goto fail;
 	}
 
+	/*
+	 * We have changed references at this point, we must abort the
+	 * transaction if anything fails.
+	 */
+	must_abort = true;
+
 	memcpy(root_item, &root->root_item, sizeof(*root_item));
 	btrfs_set_root_bytenr(root_item, eb->start);
 	btrfs_set_root_level(root_item, btrfs_header_level(eb));
@@ -789,14 +800,25 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
 
 	ret = btrfs_insert_root(trans, fs_info->tree_root,
 				&root_key, root_item);
-	BUG_ON(ret);
+	if (ret)
+		goto fail;
+
 	kfree(root_item);
 
 	reloc_root = btrfs_read_tree_root(fs_info->tree_root, &root_key);
-	BUG_ON(IS_ERR(reloc_root));
+	if (IS_ERR(reloc_root)) {
+		ret = PTR_ERR(reloc_root);
+		goto abort;
+	}
 	set_bit(BTRFS_ROOT_SHAREABLE, &reloc_root->state);
 	reloc_root->last_trans = trans->transid;
 	return reloc_root;
+fail:
+	kfree(root_item);
+abort:
+	if (must_abort)
+		btrfs_abort_transaction(trans, ret);
+	return ERR_PTR(ret);
 }
 
 /*