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>
Oh, this is just copied from @jcsp's PR, right? (Recognized the code and was worried it's a duplication.)