pml/cm: release datatype object in send completion callback

This change fixes a memory leak caused by not decreasing datatype object
reference count when the send request completes.

Signed-off-by: Wenduo Wang <wenduwan@amazon.com>

Author: wenduwan

ompi/mca/pml/cm/pml_cm_sendreq.h

@@ -455,7 +455,7 @@ do {
 #define MCA_PML_CM_HVY_SEND_REQUEST_RETURN(sendreq)                     \
     {                                                                   \
         /*  Let the base handle the reference counts */                 \
-        OMPI_DATATYPE_RETAIN(sendreq->req_send.req_base.req_datatype);  \
+        OMPI_DATATYPE_RELEASE(sendreq->req_send.req_base.req_datatype);  \
         OBJ_RELEASE(sendreq->req_send.req_base.req_comm);               \
         OMPI_REQUEST_FINI(&sendreq->req_send.req_base.req_ompi);        \
         opal_convertor_cleanup( &(sendreq->req_send.req_base.req_convertor) ); \
@@ -492,7 +492,7 @@ do {                                                                         \
 #define MCA_PML_CM_THIN_SEND_REQUEST_RETURN(sendreq)                    \
     {                                                                   \
         /*  Let the base handle the reference counts */                 \
-        OMPI_DATATYPE_RETAIN(sendreq->req_send.req_base.req_datatype);  \
+        OMPI_DATATYPE_RELEASE(sendreq->req_send.req_base.req_datatype);  \
         OBJ_RELEASE(sendreq->req_send.req_base.req_comm);               \
         OMPI_REQUEST_FINI(&sendreq->req_send.req_base.req_ompi);        \
         opal_convertor_cleanup( &(sendreq->req_send.req_base.req_convertor) ); \