ceph: fscache: add an interface to synchronize object store limit

Add an interface to explicitly synchronize object->store_limit[_l]
with inode->i_size

Tested-by: Milosz Tanski <milosz@adfin.com>
Signed-off-by: Yunchuan Wen <yunchuanwen@ubuntukylin.com>
Signed-off-by: Min Chen <minchen@ubuntukylin.com>
Signed-off-by: Li Wang <liwang@ubuntukylin.com>