Fix a bug in file ingestion (#5760)

Fix a bug in file ingestion (#5760)

Summary:
Before this PR, when the number of column families involved in a file ingestion exceeds 2, a bug in the looping logic prevents correct file number being assigned to each ingestion job.
Also skip deleting non-existing hard links during cleanup-after-failure.

Test plan (devserver)
```
$COMPILE_WITH_ASAN=1 make all
$./external_sst_file_test --gtest_filter=ExternalSSTFileTest/ExternalSSTFileTest.IngestFilesIntoMultipleColumnFamilies_*/*
$makke check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5760

Differential Revision: D17142982

Pulled By: riversand963

fbshipit-source-id: 06c1847a4e7a402647bcf28d124e70f2a0f9daf6