btrfs: find_and_setup_root error push-up

find_and_setup_root BUGs when it encounters an error from
btrfs_find_last_root, which can occur if a path can't be allocated.

This patch pushes it up to its callers where it is already handled.
Signed-off-by: Jeff Mahoney <jeffm@suse.com>

fs/btrfs/disk-io.c

@@ -1197,7 +1197,7 @@ static int __setup_root(u32 nodesize, u32 leafsize, u32 sectorsize,
 	return 0;
 }
 
-static int find_and_setup_root(struct btrfs_root *tree_root,
+static int __must_check find_and_setup_root(struct btrfs_root *tree_root,
 					    struct btrfs_fs_info *fs_info,
 					    u64 objectid,
 					    struct btrfs_root *root)
@@ -1213,7 +1213,8 @@ static int find_and_setup_root(struct btrfs_root *tree_root,
 				   &root->root_item, &root->root_key);
 	if (ret > 0)
 		return -ENOENT;
-	BUG_ON(ret);
+	else if (ret < 0)
+		return ret;
 
 	generation = btrfs_root_generation(&root->root_item);
 	blocksize = btrfs_level_size(root, btrfs_root_level(&root->root_item));