nilfs_btree_release_path() and nilfs_btree_free_path() are bound into each other
tightly. Make them into one procedure to clearify the logic and avoid some
misusages.
Signed-off-by: Li Hong <lihong.hi@gmail.com> Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>