merge with r19717, memory barrier on PPC

I run IMB exchange on two QS22 machines with r19674 and it got stucked after 256 or 512 bytes every time.
After applying r19717 the test passed, so I guess this is a essential patch.

This commit was SVN r19752.

The following SVN revision numbers were found above:
  r19674 --> open-mpi/ompi@15c47a2473
  r19717 --> open-mpi/ompi@0a765cd788

ompi/mca/btl/openib/btl_openib_component.c

@@ -2959,7 +2959,7 @@ static int progress_one_device(mca_btl_openib_device_t *device)
             uint32_t size;
             mca_btl_openib_module_t *btl = endpoint->endpoint_btl;
 
-            opal_atomic_rmb();
+            opal_atomic_mb();
 
             if(endpoint->nbo) {
                 BTL_OPENIB_FOOTER_NTOH(*frag->ftr);