mds: revoke GSHARED cap when finishing xlock

mds: revoke GSHARED cap when finishing xlock

If lock state is LOCK_XLOCKDONE, the xlocker can have GSHARED cap.
So when finishing xlock, we may need to revoke the GSHARED cap.

In most cases Locker::_finish_xlock() directly set lock state to
LOCK_LOCK or LOCK_EXCL, which hides the issue. If 'num_rdlock > 0'
or 'num_wrlock > 0' when finishing xlock, the issue reveals.
(lock get stuck in LOCK_XLOCKDONE forever)

The fix is always call Locker::_finish_xlock() when xlock count
reaches zero. _finish_xlock() checks if it can change lock state
to LOCK_EXCL immediately. If not, it uses Locker::eval_gather()
to transit lock state.

Another change of this patch is avoid changing lock state to
LOCK_LOCK directly. because lock in LOCK_XLOCK_DONE state allows
GSHARED cap, lock in LOCK_LOCK state does not.

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