Fix another TSAN lock inversion in folly::RequestContext

Summary:
TSAN complains about a lock inversion between folly/io/async/Request.cpp
lines 242 and 277.  The code as 242 was changed to use folly::acquireLocked() in
D9797715, but the code at 277 needs to be changed in the same way (except for
requesting a write lock on the child)

Reviewed By: kennyyu

Differential Revision: D10251470

fbshipit-source-id: c476a3664bfe83edbddbd402cf568eac885b9123

folly/io/async/Request.cpp

@@ -273,8 +273,9 @@ RequestContext::setShallowCopyContext() {
   auto child = std::make_shared<RequestContext>();
 
   if (parent) {
-    auto parentLock = parent->state_.rlock();
-    auto childLock = child->state_.wlock();
+    auto ret = folly::acquireLocked(as_const(parent->state_), child->state_);
+    auto& parentLock = std::get<0>(ret);
+    auto& childLock = std::get<1>(ret);
     childLock->callbackData_ = parentLock->callbackData_;
     childLock->requestData_.reserve(parentLock->requestData_.size());
     for (const auto& entry : parentLock->requestData_) {