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>
(cherry picked from commit b66ac77fa7aa3ff37804918c4308a348f239af09)