Fix race condition in fibers CacheManager
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

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly