Do nothing in spin_pause_until if spin_max is zero

Summary:
[Folly] Do nothing in `spin_pause_until` if `spin_max` is zero.

Rather than go through the loop at all, if the instruction is not to spin - encoded as `spin_max <= 0` - then return early.

Reviewed By: nbronson

Differential Revision: D6910431

fbshipit-source-id: fa94510e7ec1ebdd7bc3f1a407dcf70e1e153b40

folly/synchronization/detail/Spin.h

@@ -31,6 +31,10 @@ bool spin_pause_until(
     std::chrono::time_point<Clock, Duration> const& deadline,
     WaitOptions const& opt,
     F f) {
+  if (opt.spin_max() <= opt.spin_max().zero()) {
+    return false;
+  }
+
   auto tbegin = Clock::now();
   while (true) {
     if (f()) {