mds: fix O_TRUNC locking

When truncating a file, we should xlock the corresponding filelock.
(revoke any Fw caps from clients).

[note from sw: setattr on size also takes the filelock xlock.]

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>