tasks/mds_client_recovery: client trim its cache on reconnect

make sure CephFS client trim its cache before reconnect to the MDS.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #155 from ceph/wip-9341

tasks/mds_client_recovery: client trim its cache on reconnect

Reviewed-by: John Spray <john.spray@redhat.com>