pg_buffercache: Fix memory allocation formula

The code over-allocated the memory required for os_page_status, relying
on uint64 for its element size instead of an int, hence doubling what
was required.  This could mean quite a lot of memory if dealing with a
lot of NUMA pages.

Oversight in ba2a3c2302.

Author: David Geier <geidav.pg@gmail.com>
Discussion: https://postgr.es/m/ad0748d4-3080-436e-b0bc-ac8f86a3466a@gmail.com
Backpatch-through: 18

contrib/pg_buffercache/pg_buffercache_pages.c

@@ -383,7 +383,7 @@ pg_buffercache_os_pages_internal(FunctionCallInfo fcinfo, bool include_numa)
 
 			/* Used to determine the NUMA node for all OS pages at once */
 			os_page_ptrs = palloc0(sizeof(void *) * os_page_count);
-			os_page_status = palloc(sizeof(uint64) * os_page_count);
+			os_page_status = palloc(sizeof(int) * os_page_count);
 
 			/*
 			 * Fill pointers for all the memory pages.  This loop stores and