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>
(cherry picked from commit ff03b46509fe34f517f65f125058f60f2d64f608)

Reviewed-by: Greg Farnum <gfarnum@redhat.com>