Fix IOBufQueue.ClearAndTryReuseLargestBuffer with jemalloc

Summary: `IOBuf` capacity can be rounded up if supported by the allocator, and the round-up is done including the size of the shared info struct in `createCombined()`, so requesting 8 and 16 can return buffers with the same capacity, breaking the test.

Reviewed By: yfeldblum

Differential Revision: D33004319

fbshipit-source-id: 2bfbeaf7249e89325f9d2b11aa844c1ae1b1ceca

folly/io/test/IOBufQueueTest.cpp

@@ -130,7 +130,7 @@ TEST(IOBufQueue, Reset) {
   IOBufQueue queue(clOptions);
   queue.preallocate(8, 8);
   queue.append(SCL("Hello "));
-  queue.preallocate(16, 16);
+  queue.preallocate(128, 128);
   queue.append(SCL("World"));
   EXPECT_EQ(2, queue.front()->countChainElements());
   queue.reset();
@@ -143,13 +143,15 @@ TEST(IOBufQueue, ClearAndTryReuseLargestBuffer) {
   queue.preallocate(8, 8);
   queue.append(SCL("Hello "));
 
-  queue.preallocate(16, 16);
+  // Separate allocation sizes enough that, if they're internally rounded up,
+  // all the buffers have different capacities.
+  queue.preallocate(128, 128);
   queue.append(SCL("World "));
   // The current tail will be kept.
   const IOBuf* kept = queue.front()->prev();
 
   // The new tail is larger but cannot be reused because it's shared.
-  queue.preallocate(32, 32);
+  queue.preallocate(256, 256);
   queue.append(SCL("abc"));
   const auto shared = queue.front()->prev()->cloneOne();
   EXPECT_EQ(3, queue.front()->countChainElements());