Fix handling of MaxRead/MaxWrite export options.

If these options aren't provided, the code was already calling the
fs_maxread or fs_maxwrite FSAL ops to set the size, and if that
op returned 0, setting a default size anyway.

This export->MaxRead and export->MaxWrite are always set with a
valid value.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>