rbtree: add rb_destroy helper

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>

Conflicts:
	dog/vdi.c
	include/rbtree.h
	sheep/group.c

Conflicts were resolved by Hitoshi Mitake.
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>