ceph: fix race between page writeback and truncate

The client can receive truncate request from MDS at any time.
So the page writeback code need to get i_size, truncate_seq and
truncate_size atomically

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