mds: choose MIX state if replica of scatterlock is in MIX state

After ScatterLock::infer_state_from_strong_rejoin() set scatterlock
to LOCK_MIX state, don't change the scatterlock to other state.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: properly wake up dentry waiters after fragmenting dirfrag

When active MDS wants to fragment a replica dirfrag, it should set
the 'replay' parameter of MDCache::adjust_dir_fragments() to false.
It makes sure that CDir::merge/split wake up any dentry waiter.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: remove unused MMDSCacheRejoin::{MISSING,FULL}

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>