pml ob1: use the pre-computed size from the free lists

Based on a suggestion from George on #31806, use the pre-computed
sizes rather than duplicating the computation math (which may change
someday in the future).

cmr=v1.8.2:ticket=trac:4647

This commit was SVN r31841.

The following Trac tickets were found above:
  Ticket 4647 --> https://svn.open-mpi.org/trac/ompi/ticket/4647

ompi/mca/pml/ob1/pml_ob1_irecv.c

@@ -90,9 +90,7 @@ int mca_pml_ob1_recv(void *addr,
                      ompi_status_public_t * status)
 {
     mca_pml_ob1_recv_request_t *recvreq =
-        alloca(sizeof(mca_pml_ob1_recv_request_t) +
-               (mca_pml_ob1.max_rdma_per_request - 1) *
-               sizeof(mca_pml_ob1_com_btl_t));
+        alloca(mca_pml_base_recv_requests.fl_frag_size);
     int rc;
 
     OBJ_CONSTRUCT(recvreq, mca_pml_ob1_recv_request_t);

ompi/mca/pml/ob1/pml_ob1_isend.c

@@ -183,9 +183,7 @@ int mca_pml_ob1_send(void *buf,
     mca_bml_base_endpoint_t* endpoint = (mca_bml_base_endpoint_t*)
                                         dst_proc->proc_endpoints[OMPI_PROC_ENDPOINT_TAG_BML];
     mca_pml_ob1_send_request_t *sendreq =
-        alloca(sizeof(mca_pml_ob1_send_request_t) +
-               (mca_pml_ob1.max_rdma_per_request - 1) *
-               sizeof(mca_pml_ob1_com_btl_t));
+        alloca(mca_pml_base_send_requests.fl_frag_size);
     int16_t seqn;
     int rc;