Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9685

Our TSAN reports a race condition as follows when running test
```
gtest-parallel -r 100 ./external_sst_file_test --gtest_filter=ExternalSSTFileTest.MultiThreaded
```
leads to the following

```
WARNING: ThreadSanitizer: data race (pid=2683148)
  Write of size 1 at 0x556fede63340 by thread T7:
    #0 rocksdb::(anonymous namespace)::PosixFileSystem::OpenWritableFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::FileOptions const&, bool, std::unique_ptr<rocksdb::FSWritableFile, std::default_delete<rocksdb::FSWritableFile> >*, rocksdb::IODebugContext*) internal_repo_rocksdb/repo/env/fs_posix.cc:334 (external_sst_file_test+0xb61ac4)
    #1 rocksdb::(anonymous namespace)::PosixFileSystem::ReopenWritableFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::FileOptions const&, std::unique_ptr<rocksdb::FSWritableFile, std::default_delete<rocksdb::FSWritableFile> >*, rocksdb::IODebugContext*) internal_repo_rocksdb/repo/env/fs_posix.cc:382 (external_sst_file_test+0xb5ba96)
    #2 rocksdb::CompositeEnv::ReopenWritableFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::unique_ptr<rocksdb::WritableFile, std::default_delete<rocksdb::WritableFile> >*, rocksdb::EnvOptions const&) internal_repo_rocksdb/repo/env/composite_env.cc:334 (external_sst_file_test+0xa6ab7f)
    #3 rocksdb::EnvWrapper::ReopenWritableFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::unique_ptr<rocksdb::WritableFile, std::default_delete<rocksdb::WritableFile> >*, rocksdb::EnvOptions const&) internal_repo_rocksdb/repo/include/rocksdb/env.h:1428 (external_sst_file_test+0x561f3e)
Previous read of size 1 at 0x556fede63340 by thread T4:
    #0 rocksdb::(anonymous namespace)::PosixFileSystem::OpenWritableFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::FileOptions const&, bool, std::unique_ptr<rocksdb::FSWritableFile, std::default_delete<rocksdb::FSWritableFile> >*, rocksdb::IODebugContext*) internal_repo_rocksdb/repo/env/fs_posix.cc:328 (external_sst_file_test+0xb61a70)
    #1 rocksdb::(anonymous namespace)::PosixFileSystem::ReopenWritableFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator
...
```

Fix by making sure the following block gets executed only once:
```
      if (!checkedDiskForMmap_) {
        // this will be executed once in the program's lifetime.
        // do not use mmapWrite on non ext-3/xfs/tmpfs systems.
        if (!SupportsFastAllocate(fname)) {
          forceMmapOff_ = true;
        }
        checkedDiskForMmap_ = true;
      }
```

Reviewed By: pdillinger

Differential Revision: D34780308

fbshipit-source-id: b761f66b24c8b5b8389d86ea371c8542b8d869d5