Cluster: fixed MF condition in clusterHandleSlaveFailover().
Cluster: fixed MF condition in clusterHandleSlaveFailover().

For manual failover we need a manual failover in progress, and that
mf_can_start is true (master offset received and matched).

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis