Improve GIN index build's tracking of memory usage by using

Improve GIN index build's tracking of memory usage by using
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.