btrfs: use BTRFS_PATH_AUTO_FREE in populate_free_space_tree()

This is the trivial pattern for path auto free, initialize at the
beginning and free at the end with simple goto -> return conversions.
This applies to both path and path2.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>