mds: fix scattered wrlock rejoin

mds: fix scattered wrlock rejoin

If unstable scatter lock is encountered when handling weak cache
rejoin, don't remove the recovering MDS from the scatter lock's
gather list. The reason is the recovering MDS may hold rejoined
wrlock on the scatter lock. (Rejoined wrlocks were created when
handling strong cache rejoins from survivor MDS)

When composing cache rejoin ack, if the recovering MDS is in lock's
gather list, set lock state of the recovering MDS to a compatible
unstable stable.

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