GIN index build's allocatedMemory counter needs to be long, not uint32.

GIN index build's allocatedMemory counter needs to be long, not uint32.
Else, in a 64-bit machine with maintenance_work_mem set to above 4Gb,
the counter overflows and we never recognize having reached the
maintenance_work_mem limit.  I believe this explains out-of-memory
failure recently reported by Sean Davis.

This is a bug, so backpatch to 8.2.