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>
Merge pull request #2458 from ceph/wip-9341

client: trim unused inodes before reconnecting to recovering MDS

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: John Spray <john.spray@redhat.com>