Fix race condition in fibers CacheManager

Summary:
The non-atomic field `inUse_` is protected by a lock in `getStackCache`, but not in `giveBack`. This
is a data race and it is caught by TSAN.

Reviewed By: yfeldblum

Differential Revision: D17109710

fbshipit-source-id: 7c0d5d948740516422efca0895a702566c804bb4

folly/fibers/GuardPageAllocator.cpp

@@ -272,6 +272,7 @@ class CacheManager {
   friend class StackCacheEntry;
 
   void giveBack(std::unique_ptr<StackCache> /* stackCache_ */) {
+    std::lock_guard<folly::SpinLock> lg(lock_);
     assert(inUse_ > 0);
     --inUse_;
     /* Note: we can add a free list for each size bucket