cephfs: fix mount point break off problem after mds switch occured

cephfs: fix mount point break off problem after mds switch occured

The hot-standby become active as we expected but the mount piont broken strangely
when the active mds is down. The root reason is the new mds use last_cap_renews
decoded from ESesson::replay in find_idle_sessions and wrongly killed the session.
Maybe we should reset session->last_cap_renew to the current time when server send
OPEN to client in reconnect stage.

Fixes: http://tracker.ceph.com/issues/19437
Signed-off-by: Guan yunfei <yunfei.guan@xtaotech.com>