Allow the BTL to release the descriptor. In fact the only thing the PML

needs is to be involved in the RMA completion process, which is insured
by the MCA_BTL_DES_SEND_ALWAYS_CALLBACK flag. Fixes trac:1875.

This commit was SVN r20983.

The following Trac tickets were found above:
  Ticket 1875 --> https://svn.open-mpi.org/trac/ompi/ticket/1875

ompi/mca/pml/ob1/pml_ob1_recvreq.c

@@ -362,7 +362,7 @@ int mca_pml_ob1_recv_request_get_frag( mca_pml_ob1_rdma_frag_t* frag )
                               MCA_BTL_NO_ORDER,
                               0,
                               &frag->rdma_length,
-                              0,  /* always call the callback, PML ownership */
+                              MCA_BTL_DES_FLAGS_BTL_OWNERSHIP | MCA_BTL_DES_SEND_ALWAYS_CALLBACK,
                               &descriptor );
     if( OPAL_UNLIKELY(NULL == descriptor) ) {
         frag->rdma_length = save_size;