individual read/write operations exceeding 2GB fail in ompio
due to improper conversions from size_t to int in two different
locations. This commit fixes an issue reported by Richard Warren
from the HDF5 group.
Fixes Issue #7045
Cherry-picked from commit a130f569df6badebe639d17c0a1a0cb79a596094
Signed-off-by: Edgar Gabriel <egabriel@central.uh.edu>