BUG #8034: pg_buffercache gets invalid memory alloc request size with very large shared memory buffers

The following bug has been logged on the website:

Bug reference: 8034
Logged by: Devin Ben-Hur
Email address: dbenhur(at)whitepages(dot)com
PostgreSQL version: 9.2.3
Operating system: Ubuntu Precise
Description:

When a very large shared buffer pool (~480GB) is used with postgresql,
pg_buffercache contrib module gets an allocation error trying to Allocate
NBuffers worth of BufferCachePagesRec records:

https://github.com/postgres/postgres/blob/REL9_2_3/contrib/pg_buffercache/pg_buffercache_pages.c#L101-L102

The requested allocation exceeds the 1GB limitation imposed by
AllocSizeIsValid macro:
https://github.com/postgres/postgres/blob/REL9_2_3/src/include/utils/memutils.h#L40-L43

Reproduce:
1) acquire server with half terabyte of memory
2) tweak OS settings to allow large shared memory
3) set postgresql.conf: shared_buffers = 400GB
4) CREATE EXTENSION pg_buffercache;
5) SELECT * FROM pg_buffercache LIMIT 1;