osdc/ObjectCacher: finish contexts after dropping object reference

The context to finish can be class C_Client_PutInode, which may drop
inode's last reference. So we should first drop object's reference,
then finish contexts.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #590 from ceph/wip-6284

osdc/ObjectCacher: finish contexts after dropping object reference

Tested: ubuntu@teuthology:/a/sage-2013-09-19_19:40:56-fs-wip-6284-testing-basic-plana
Reviewed-by: Sage Weil <sage@inktank.com>