cifs: cancel_delayed_work() + flush_scheduled_work() -> cancel_delayed_work_sync()

flush_scheduled_work() is going away.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>