Avoid integer overflow while testing wal_skip_threshold condition.

smgrDoPendingSyncs had two distinct risks of integer overflow while
deciding which way to ensure durability of a newly-created relation.
First, it accumulated the total size of all forks in a variable of
type BlockNumber (uint32).  While we restrict an individual fork's
size to fit in that, I don't believe there's such a restriction on
all of them added together.  Second, it proceeded to multiply the
sum by BLCKSZ, which most certainly could overflow a uint32.

(The exact expression is total_blocks * BLCKSZ / 1024.  The
compiler might choose to optimize that to total_blocks * 8,
which is not at quite as much risk of overflow as a literal
reading would be, but it's still wrong.)

If an overflow did occur it could lead to a poor choice to
shove a very large relation into WAL instead of fsync'ing it.
This wouldn't be fatal, but it could be inefficient.

Change total_blocks to uint64 which should be plenty, and
rearrange the comparison calculation to be overflow-safe.

I noticed this while looking for ramifications of the proposed
change in MAX_KILOBYTES.  It's not entirely clear to me why
wal_skip_threshold is limited to MAX_KILOBYTES in the
first place, but in any case this code is unsafe regardless
of the range of wal_skip_threshold.

Oversight in c6b9204 which introduced wal_skip_threshold,
so back-patch to v13.

Discussion: https://postgr.es/m/1a01f0-66ec2d80-3b-68487680@27595217
Backpatch-through: 13
(cherry picked from commit 1e25cdb214543d8b661cf01bbdb6f8e2b1a0381e)

src/backend/catalog/storage.c

@@ -783,7 +783,7 @@ smgrDoPendingSyncs(bool isCommit, bool isParallelWorker)
 	{
 		ForkNumber	fork;
 		BlockNumber nblocks[MAX_FORKNUM + 1];
-		BlockNumber total_blocks = 0;
+		uint64		total_blocks = 0;
 		SMgrRelation srel;
 
 		srel = smgropen(pendingsync->rlocator, INVALID_PROC_NUMBER);
@@ -827,7 +827,7 @@ smgrDoPendingSyncs(bool isCommit, bool isParallelWorker)
 		 * main fork is longer than ever but FSM fork gets shorter.
 		 */
 		if (pendingsync->is_truncated ||
-			total_blocks * BLCKSZ / 1024 >= wal_skip_threshold)
+			total_blocks >= wal_skip_threshold * (uint64) 1024 / BLCKSZ)
 		{
 			/* allocate the initial array, or extend it, if needed */
 			if (maxrels == 0)