pre-allocate freeList

Summary: We know ahead of time the size of this list, so may as well pre-allocate space for it

Reviewed By: yfeldblum

Differential Revision: D22838412

fbshipit-source-id: 7d70495fe0910e65dfea49fe7baf675308ab9331

folly/fibers/GuardPageAllocator.cpp

@@ -74,6 +74,7 @@ class StackCache {
     storage_ = reinterpret_cast<unsigned char*>(p);
 
     /* Protect the bottommost page of every stack allocation */
+    freeList_.reserve(kNumGuarded);
     for (size_t i = 0; i < kNumGuarded; ++i) {
       auto allocBegin = storage_ + allocSize_ * i;
       freeList_.emplace_back(allocBegin, /* protected= */ false);