PreservedMarks verification code fails

We get assertion failures like:
# Internal Error (src/share/vm/gc/shared/preservedMarks.inline.hpp:68), pid=25548, tid=25556
# assert(total_size == total_size_before) failed: total_size = 0 before = 9512

because the code to update total_size updates the total_size_addr pointer and not the value.