Use correct GetDatum() in pg_relation_check_pages()

UInt32GetDatum() was getting used, while the result needs
Int64GetDatum().  Oversight in f2b8839.

Per buildfarm member florican.

Discussion: https://postgr.es/m/1226629.1603859189@sss.pgh.pa.us

src/backend/utils/adt/pagefuncs.c

@@ -214,7 +214,7 @@ check_relation_fork(TupleDesc tupdesc, Tuplestorestate *tupstore,
 		memset(nulls, 0, sizeof(nulls));
 
 		values[i++] = CStringGetTextDatum(path);
-		values[i++] = UInt32GetDatum(blkno);
+		values[i++] = Int64GetDatum((int64) blkno);
 
 		Assert(i == PG_CHECK_RELATION_COLS);