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>
To invalidate a dentry, the fuse kernel driver need to take its parent directory's i_mutex. The i_mutex can be hold by a hang operation. So we can't wait.