fix transaction rollback in db_stress TestMultiGet (#6873)

Summary:
There were further uses of `txn` after `RollbackTxn(txn)` leading to
stress test errors. Moved the rollback to the end of the function.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6873

Test Plan:
found a command from the crash test that previously failed immediately under TSAN; verified now it succeeds.
```
./db_stress --acquire_snapshot_one_in=10000 --allow_concurrent_memtable_write=1 --avoid_flush_during_recovery=0 --avoid_unnecessary_blocking_io=1 --block_size=16384 --bloom_bits=222.913637674 --bottommost_compression_type=none --cache_index_and_filter_blocks=1 --cache_size=1048576 --checkpoint_one_in=0 --checksum_type=kCRC32c --clear_column_family_one_in=0 --compact_files_one_in=1000000 --compact_range_one_in=1000000 --compaction_style=1 --compaction_ttl=0 --compression_max_dict_bytes=0 --compression_parallel_threads=1 --compression_type=zstd --compression_zstd_max_train_bytes=0 --continuous_verification_interval=0 --db=/dev/shm/rocksdb/rocksdb_crashtest_whitebox --db_write_buffer_size=1048576 --delpercent=5 --delrangepercent=0 --destroy_db_initially=0 --disable_wal=0 --enable_pipelined_write=0 --flush_one_in=1000000 --format_version=5 --get_current_wal_file_one_in=0 --get_live_files_one_in=1000000 --get_sorted_wal_files_one_in=0 --index_block_restart_interval=12 --index_type=2 --key_len_percent_dist=1,30,69 --level_compaction_dynamic_level_bytes=True --log2_keys_per_lock=22 --long_running_snapshots=0 --max_background_compactions=20 --max_bytes_for_level_base=10485760 --max_key=1000000 --max_key_len=3 --max_manifest_file_size=1073741824 --max_write_batch_group_size_bytes=1048576 --max_write_buffer_number=3 --memtablerep=skip_list --mmap_read=1 --mock_direct_io=False --nooverwritepercent=1 --num_levels=1 --open_files=100 --ops_per_thread=200000 --partition_filters=1 --pause_background_one_in=1000000 --periodic_compaction_seconds=0 --prefixpercent=5 --progress_reports=0 --read_fault_one_in=0 --readpercent=45 --recycle_log_file_num=0 --reopen=20 --snapshot_hold_ops=100000 --subcompactions=4 --sync=0 --sync_fault_injection=False --target_file_size_base=2097152 --target_file_size_multiplier=2 --test_batches_snapshots=0 --txn_write_policy=1 --unordered_write=1 --use_block_based_filter=0 --use_direct_io_for_flush_and_compaction=0 --use_direct_reads=0 --use_full_merge_v1=1 --use_merge=1 --use_multiget=1 --use_txn=1 --verify_checksum=1 --verify_checksum_one_in=1000000 --verify_db_one_in=100000 --write_buffer_size=4194304 --write_dbid_to_manifest=0 --writepercent=35
```

Reviewed By: pdillinger

Differential Revision: D21708338

Pulled By: ajkr

fbshipit-source-id: dcf55cddee0a14f429a75e7a8a505acf8025f2b1

db_stress_tool/no_batched_ops_stress.cc

@@ -326,7 +326,6 @@ class NonBatchedOpsStressTest : public StressTest {
 #ifndef ROCKSDB_LITE
       txn->MultiGet(readoptionscopy, cfh, num_keys, keys.data(), values.data(),
                     statuses.data());
-      RollbackTxn(txn);
 #endif
     }
 
@@ -422,6 +421,11 @@ class NonBatchedOpsStressTest : public StressTest {
     if (readoptionscopy.snapshot) {
       db_->ReleaseSnapshot(readoptionscopy.snapshot);
     }
+    if (use_txn) {
+#ifndef ROCKSDB_LITE
+      RollbackTxn(txn);
+#endif
+    }
     return statuses;
   }