Fix page-level checksum calculation in checksum_impl.h

Issue introduced by me, as of 00651743.

Reported-by: David Steele
Discussion: https://postgr.es/m/1cf30561-7dad-dc6e-9fc3-5c456948cfeb@pgmasters.net

src/include/storage/checksum_impl.h

@@ -211,5 +211,5 @@ pg_checksum_page(char *page, BlockNumber blkno)
 	 * Reduce to a uint16 (to fit in the pd_checksum field) with an offset of
 	 * one. That avoids checksums of zero, which seems like a good idea.
 	 */
-	return (uint16) ((checksum % 65536) + 1);
+	return (uint16) ((checksum % 65535) + 1);
 }