Call PMPI_Finalize() before unmount the client

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Performance enhancement (non-breaking change which improves efficiency)
[ ] Code cleanup (non-breaking change which makes code smaller or more readable)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Testing (addition of new tests or update to current tests)
[ ] Documentation (a change to man pages or other documentation)

LLNL / UnifyFS