Convert implicit cast from `long` to `double` to a `static_cast`

Summary:
This code implicitly casts a `long` to `double` but a `double`
can not accurately represent an integer above `2^53` as
the mantissa only stores that many digits. At `2^53 + 1`
`double`s increment by `2`. Thus this cast is lossy. Though,
casting a single value `LONG_MAX` to a `double` is and
being off by one doesn't cause any harm here.

Reviewed By: ispeters

Differential Revision: D25654006

fbshipit-source-id: 3d84092f1786d7879dd48d73af8edbbc80bb3585

folly/futures/Retrying.h

@@ -207,7 +207,7 @@ Duration retryingJitteredExponentialBackoffDur(
   auto dist = std::normal_distribution<double>(0.0, jitter_param);
   auto jitter = std::exp(dist(rng));
   auto backoff_rep = jitter * backoff_min.count() * std::pow(2, n - 1);
-  if (UNLIKELY(backoff_rep >= std::numeric_limits<Duration::rep>::max())) {
+  if (UNLIKELY(backoff_rep >= static_cast<double>(backoff_max.count()))) {
     return std::max(backoff_min, backoff_max);
   }
   auto backoff = Duration(Duration::rep(backoff_rep));