GetMemoryChunkSpace, not just the palloc request size.  This brings the
allocatedMemory counter close enough to reality (as measured by
MemoryContextStats printouts) that I think we can get rid of the arbitrary
factor-of-2 adjustment that was put into the code initially.  Given the
sensitivity of GIN build to work memory size, not using as much of work
memory as we're allowed to seems a pretty bad idea.