Set filesystem constructor parameter for FilePrefetchBuffer inside PrefetchTail

[archang19]

Summary

After #13118 was merged, I did some investigation to see whether the file system buffer reuse code was actually being used.

The good news is that I was able to see from the CPU profiling results that my code is getting invoked through the warm storage stress tests.

The bad news is that most of the time, the optimization is not being used, so we end up going through the regular old RandomAccessFileReader::Read path.

Here is the entire function call chain up to FilePrefetchBuffer::Read

1. rocksdb::DB::MultiGet
2. rocksdb::DBImpl::MultiGet
3. rocksdb::DBImpl::MultiGetCommon
4. rocksdb::DBImpl::MultiGetImpl
5. rocksdb::Version::MultiGet
6. rocksdb::Version::MultiGetFromSST
7. rocksdb::TableCache::MultiGet
8. rocksdb::TableCache::FindTable
9. rocksdb::TableCache::GetTableReader
10. rocksdb::BlockBasedTableFactory::NewTableReader
11. rocksdb::BlockBasedTable::Open
12. rocksdb::BlockBasedTable::PrefetchTail
13. rocksdb::FilePrefetchBuffer::Prefetch
14. rocksdb::FilePrefetchBuffer::Read

At this point, we split into rocksdb::RandomAccessFileReader::Read and
rocksdb::FilePrefetchBuffer::FSBufferDirectRead. FSBufferDirectRead gets called <3% of the time.

I think the root cause is that the FileSystem* fs parameter is not getting passed into the FilePrefetchBuffer constructor. When fs is nullptr, UseFSBuffer() will always return false and we do not end up calling FSBufferDirectRead.

Luckily, it does not seem like there are too many places I need to change. BlockBasedTable resets its prefetch_buffer in 3 separate places. When it disables the prefetch buffer (2/3 of the instances), we don't care about whether the fs parameter is there. This PR is addressing the third instance, where it is not trying to disable the buffer.

Note that there is another method, PrefetchBufferCollection::GetOrCreatePrefetchBuffer that creates new FilePrefetchBuffers without the fs parameter. This method gets called by compaction_iterator and merge_helper. I think we can address this in a subsequent PR:

1. Each of these changes effectively "unlocks" the buffer reuse feature. Separating the changes would be helpful when I look at the profiling results again, since I can isolate what impact this PR had on the percentage of time that rocksdb::FilePrefetchBuffer::FSBufferDirectRead was invoked.
2. I still need to look into what exactly I would need to changes I need to make to PrefetchBufferCollection
3. This code seems to be for blob prefetching in particular, and I don't think it has the biggest ROI anyways.

      const Status s = blob_fetcher_->FetchBlob(
          user_key(), blob_index, prefetch_buffer, &blob_value_, &bytes_read);

4. I am not sure if the current benchmark I am using for warm storage exercises this blob prefetching code, so I may need to find another way to assess the performance impact.

Test Plan

The existing unit test coverage guards against obvious bugs. I ran another set of performance tests to confirm there were no regressions in CPU utilization.

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