sheep: remove invalid usage of current_vnode_info

The variable current_vnode_info should be treated with the functions
get_vnode_info() and put_vnode_info(). But some part of group.c didn't
follow this rule. This patch fixes the clear problem in
update_cluster_info(), sd_leave_handler() and kick_node_recover().

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: operate vnode info between get_vnode_info() and put_vnode_info()

vnode_info should be obtained by get_vnode_info() and released by
put_vnode_info(). Some part of group.c don't follow the manner. It
doesn't cause a serious problem practically but is not good for coding
convention. This patch does refactoring for this problem.

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