Summary:
Before this fix, atomic flush codepath may hit an assertion failure on a specific failure case.
If all flush jobs within an atomic flush succeed (they do not write to MANIFEST), but batch writing version edits to MANIFEST fails, then `cfd->imm()->RollbackMemTableFlush()` will be called twice, and the second invocation hits assertion failure `assert(m->flush_in_progress_)` since the first invocation resets the variable `flush_in_progress_` to false already.
Test plan (dev server):
```
./db_flush_test --gtest_filter=DBAtomicFlushTest/DBAtomicFlushTest.RollbackAfterFailToInstallResults
make check
```
Both must succeed.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6385
Differential Revision: D19782943
Pulled By: riversand963
fbshipit-source-id: 84e1592625e729d1b70fdc8479959387a74cb121
f361cedf06