osc/portals4: Put/Get splitting because Portals4 may restrict sizes

              + take into account the lower bound of the datatype in each portals4 call

Author: PDeveze

ompi/mca/osc/portals4/osc_portals4.h

@@ -53,6 +53,7 @@ struct ompi_osc_portals4_component_t {
     ptl_size_t matching_atomic_max;
     ptl_size_t matching_fetch_atomic_max;
     ptl_size_t matching_atomic_ordered_size;
+    ptl_size_t ptl_max_msg_size; /* max size given by portals (cf PtlNIInit) */
     ptl_uid_t uid;
     opal_mutex_t lock;
     opal_condition_t cond;