Use Size instead of int64 to track allocated memory

Commit 5dd7fc1519 added block-level memory accounting, but used int64 variable to
track the amount of allocated memory. That is incorrect, because we have Size for
exactly these purposes, but it was mostly harmless until c477f3e449 which changed
how we handle with repalloc() when downsizing the chunk. Previously we've ignored
these cases and just kept using the original chunk, but now we need to update the
accounting, and the code was doing this:

    context->mem_allocated += blksize - oldblksize;

Both blksize and oldblksize are Size (so unsigned) which means the subtraction
underflows, producing a very high positive value. On 64-bit platforms (where Size
has the same size as mem_alllocated) this happens to work because the result wraps
to the right value, but on (some) 32-bit platforms this fails.

This fixes two things - it changes mem_allocated (and related variables) to Size,
and it splits the update to two separate steps, to prevent any underflows.

Discussion: https://www.postgresql.org/message-id/15151.1570163761%40sss.pgh.pa.us

src/backend/utils/mmgr/aset.c

@@ -1154,7 +1154,9 @@ AllocSetRealloc(MemoryContext context, void *pointer, Size size)
 			return NULL;
 		}
 
-		context->mem_allocated += blksize - oldblksize;
+		/* updated separately, not to underflow when (oldblksize > blksize) */
+		context->mem_allocated -= oldblksize;
+		context->mem_allocated += blksize;
 
 		block->freeptr = block->endptr = ((char *) block) + blksize;
 
@@ -1427,7 +1429,7 @@ AllocSetCheck(MemoryContext context)
 	const char *name = set->header.name;
 	AllocBlock	prevblock;
 	AllocBlock	block;
-	int64		total_allocated = 0;
+	Size		total_allocated = 0;
 
 	for (prevblock = NULL, block = set->blocks;
 		 block != NULL;