Remove use of pg_memory_is_all_zeros() in bufpage.c

After a closer lookup, this makes the all-zero check of the page more
expensive, so let's remove the new function call in bufpage.c.  The
maths of the check were also incorrect, checking that the page was full
of zeros only for the first 1kB.

This brings back the code to the state it was at 49d6c7d8da.

Per discussion with David Rowley and Bertrand Drouvot.

Discussion: https://postgr.es/m/CAApHDvrXzPAr3FxoBuB7b3D-okNoNA2jxLun1rW8Yw5wkbqusw@mail.gmail.com

src/backend/storage/page/bufpage.c

@@ -89,8 +89,10 @@ PageIsVerifiedExtended(Page page, BlockNumber blkno, int flags)
 {
 	PageHeader	p = (PageHeader) page;
 	size_t	   *pagebytes;
+	int			i;
 	bool		checksum_failure = false;
 	bool		header_sane = false;
+	bool		all_zeroes = false;
 	uint16		checksum = 0;
 
 	/*
@@ -124,9 +126,18 @@ PageIsVerifiedExtended(Page page, BlockNumber blkno, int flags)
 	}
 
 	/* Check all-zeroes case */
+	all_zeroes = true;
 	pagebytes = (size_t *) page;
+	for (i = 0; i < (BLCKSZ / sizeof(size_t)); i++)
+	{
+		if (pagebytes[i] != 0)
+		{
+			all_zeroes = false;
+			break;
+		}
+	}
 
-	if (pg_memory_is_all_zeros(pagebytes, (BLCKSZ / sizeof(size_t))))
+	if (all_zeroes)
 		return true;
 
 	/*