Revert "sheep: remove invalid usage of current_vnode_info"

This reverts commit b82632b47978315a50e9ba0bbad59f56453f63f5.

The commit replaced main_thread_get() with get_vnode_info(). But the change was
invalid because if we use get_vnode_info() for old_vnode_info, the reference
count never reaches to 0. It causes resource leak. This patch reverts the
commit.
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
[ Refine some comment - Yuan ]
Signed-off-by: Liu Yuan <namei.unix@gmail.com>