Fix race condition for accessing file size in TestFSWritableFile (#12312

)

Summary:
Fix a race condition reported by thread sanitizer for accessing an underlying file's size from `TestFSWritableFile`.

Pull Request resolved: #12312

Test Plan:
COMPILE_WITH_TSAN=1 make -j10 transaction_test
./transaction_test --gtest_filter="DBAsBaseDB/TransactionTest.UnlockWALStallCleared/4" --gtest_repeat=100

Reviewed By: pdillinger

Differential Revision: D53235231

Pulled By: jowlyzhang

fbshipit-source-id: 35133cd97f8cbb48746ca3b42baeedecb36beb7b

utilities/fault_injection_fs.h

@@ -98,6 +98,7 @@ class TestFSWritableFile : public FSWritableFile {
 
   virtual uint64_t GetFileSize(const IOOptions& options,
                                IODebugContext* dbg) override {
+    MutexLock l(&mutex_);
     return target_->GetFileSize(options, dbg);
   }