mds: introduce XSYN to SYNC lock state transition

mds: introduce XSYN to SYNC lock state transition

If lock is in XSYN state, Locker::simple_sync() firstly try changing
lock state to EXCL. If it fail to change lock state to EXCL, it just
returns. So Locker::simple_sync() does not guarantee the lock state
eventually changes to SYNC. This issue can cause replica that requests
read lock hang. The fix is introduce an intermediate state for XSYN
to SYNC transition.

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