qht: Fix threshold rate calculation

tests/qht-bench.c:287:29: error: implicit conversion from 'unsigned long'
  to 'double' changes value from 18446744073709551615
  to 18446744073709551616 [-Werror,-Wimplicit-int-float-conversion]
        *threshold = rate * UINT64_MAX;
                          ~ ^~~~~~~~~~

Fix this by splitting the 64-bit constant into two halves,
each of which is individually perfectly representable, the
sum of which produces the correct arithmetic result.

This is very likely just a sticking plaster over some underlying
incorrect code, but it will suppress the warning for the moment.

Cc: Emilio G. Cota <[email protected]>
Reported-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>

Author: rth7680

tests/qht-bench.c

@@ -284,7 +284,8 @@ static void do_threshold(double rate, uint64_t *threshold)
     if (rate == 1.0) {
         *threshold = UINT64_MAX;
     } else {
-        *threshold = rate * UINT64_MAX;
+        *threshold = (rate * 0xffff000000000000ull)
+                   + (rate * 0x0000ffffffffffffull);
     }
 }