btrfs: handle extent corruption with select_one_root properly

In corruption cases we could have paths from a block up to no root at
all, and thus we'll BUG_ON(!root) in select_one_root.  Handle this by
adding an ASSERT() for developers, and returning an error for normal
users.
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

@@ -2218,7 +2218,13 @@ struct btrfs_root *select_one_root(struct btrfs_backref_node *node)
 		cond_resched();
 		next = walk_up_backref(next, edges, &index);
 		root = next->root;
-		BUG_ON(!root);
+
+		/*
+		 * This can occur if we have incomplete extent refs leading all
+		 * the way up a particular path, in this case return -EUCLEAN.
+		 */
+		if (!root)
+			return ERR_PTR(-EUCLEAN);
 
 		/* No other choice for non-shareable tree */
 		if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
@@ -2608,8 +2614,15 @@ static int relocate_tree_block(struct btrfs_trans_handle *trans,
 
 	BUG_ON(node->processed);
 	root = select_one_root(node);
-	if (root == ERR_PTR(-ENOENT)) {
+	if (IS_ERR(root)) {
+		ret = PTR_ERR(root);
+
+		/* See explanation in select_one_root for the -EUCLEAN case. */
+		ASSERT(ret == -ENOENT);
+		if (ret == -ENOENT) {
+			ret = 0;
 			update_processed_blocks(rc, node);
+		}
 		goto out;
 	}