btrfs: unify the error handling of btrfs_read_buffer()

There is one oddball error handling of btrfs_read_buffer():

	ret = btrfs_read_buffer(tmp, gen, parent_level - 1, &first_key);
	if (!ret) {
		*eb_ret = tmp;
		return 0;
	}
	free_extent_buffer(tmp);
	btrfs_release_path(p);
	return -EIO;

While all other call sites check the error first.  Unify the behavior.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>