Move a function to critical section (#4752)

Summary:
Test plan
```
$make clean && make -j32 all check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4752

Differential Revision: D13344705

Pulled By: riversand963

fbshipit-source-id: fc3a43174d09d70ccc2b09decd78e1da1b6ba9d1

db/db_impl.cc

@@ -3131,20 +3131,19 @@ Status DBImpl::IngestExternalFile(
                                &need_flush);
       if (status.ok() && need_flush) {
         if (immutable_db_options_.atomic_flush) {
-          mutex_.Unlock();
           autovector<ColumnFamilyData*> cfds;
           SelectColumnFamiliesForAtomicFlush(&cfds);
+          mutex_.Unlock();
           status = AtomicFlushMemTables(cfds, FlushOptions(),
                                         FlushReason::kExternalFileIngestion,
                                         true /* writes_stopped */);
-          mutex_.Lock();
         } else {
           mutex_.Unlock();
           status = FlushMemTable(cfd, FlushOptions(),
                                  FlushReason::kExternalFileIngestion,
                                  true /* writes_stopped */);
-          mutex_.Lock();
         }
+        mutex_.Lock();
       }
     }
 

db/db_impl_compaction_flush.cc

@@ -588,7 +588,9 @@ Status DBImpl::CompactRange(const CompactRangeOptions& options,
     fo.allow_write_stall = options.allow_write_stall;
     if (immutable_db_options_.atomic_flush) {
       autovector<ColumnFamilyData*> cfds;
+      mutex_.Lock();
       SelectColumnFamiliesForAtomicFlush(&cfds);
+      mutex_.Unlock();
       s = AtomicFlushMemTables(cfds, fo, FlushReason::kManualCompaction,
                                false /* writes_stopped */);
     } else {