Fix incorrect alignment for Fiber::userBuffer_

Summary:
D88106410 replaced `std::aligned_storage` with `folly::aligned_storage_t` across folly. In `Fiber.h`, `userBuffer_` was incorrectly changed from `std::aligned_storage<kUserBufferSize>::type` (default alignment) to `folly::aligned_storage_t<kUserBufferSize, cacheline_align_v>` (64-byte cacheline alignment). The `cacheline_align_v` was not in the original code — it was mistakenly copied from the adjacent `Buffer` typedef.

This removes the spurious `cacheline_align_v` argument so `folly::aligned_storage_t<kUserBufferSize>` uses its default alignment, matching the original `std::aligned_storage<256>::type` behavior.

Reviewed By: dmm-fb, yfeldblum

Differential Revision: D95415011

fbshipit-source-id: a6bef72cd1caf3ee7058157a61fddceb555560de

Author: generatedunixname893464919433493

folly/fibers/Fiber.h

@@ -161,7 +161,7 @@ class Fiber {
   folly::AtomicIntrusiveLinkedListHook<Fiber> nextRemoteReady_;
 
   static constexpr size_t kUserBufferSize = 256;
-  folly::aligned_storage_t<kUserBufferSize, cacheline_align_v> userBuffer_;
+  folly::aligned_storage_t<kUserBufferSize> userBuffer_;
 
   void* getUserBuffer();