client: trim unused inodes before reconnecting to recovering MDS

So the recovering MDS does not need to fetch these ununsed inodes during
cache rejoin. This may reduce MDS recovery time.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
I'm fine with hoping. Just wondered if we wanted to set a timeout (a couple seconds or something) and not send a rejoin message to the MDS until either the timeout or we'd successfully dropped enough inodes.