Summary:
When investigating https://github.com/facebook/rocksdb/issues/6666, we encounter an error for sst_dump to dump an ingested SST file with global seqno.
```
Corruption: An external sst file with version 2 have global seqno property with value ��/, while largest seqno in the file is 0）
```
Same as https://github.com/facebook/rocksdb/pull/5097, it is due to SstFileReader don't know the largest seqno of a file, it will fail this check when it open a file with global seqno. ca89ac2ba9/table/block_based_table_reader.cc (L730)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6673
Test Plan: run it manually
Reviewed By: cheng-chang
Differential Revision: D20937546
Pulled By: ajkr
fbshipit-source-id: c3fd04d60916a738533ee1885f3ea844669a9479
c8c739a877