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