Incorrect Universal Compaction reason

Summary:
While writing tests for dynamic Universal Compaction options, I found that the compaction reasons we set for size-ratio based and sorted-run based universal compactions are swapped with each other. Fixed it.
Closes https://github.com/facebook/rocksdb/pull/3412

Differential Revision: D6820540

Pulled By: sagar0

fbshipit-source-id: 270a188968ba25b2c96a8339904416c4c87ff5b3

db/compaction_picker_universal.cc

@@ -603,9 +603,9 @@ Compaction* UniversalCompactionPicker::PickCompactionToReduceSortedRuns(
 
   CompactionReason compaction_reason;
   if (max_number_of_files_to_compact == UINT_MAX) {
-    compaction_reason = CompactionReason::kUniversalSortedRunNum;
-  } else {
     compaction_reason = CompactionReason::kUniversalSizeRatio;
+  } else {
+    compaction_reason = CompactionReason::kUniversalSortedRunNum;
   }
   return new Compaction(
       vstorage, ioptions_, mutable_cf_options, std::move(inputs), output_level,

db/listener_test.cc

@@ -528,7 +528,7 @@ TEST_F(EventListenerTest, CompactionReasonUniversal) {
 
   ASSERT_GT(listener->compaction_reasons_.size(), 0);
   for (auto compaction_reason : listener->compaction_reasons_) {
-    ASSERT_EQ(compaction_reason, CompactionReason::kUniversalSortedRunNum);
+    ASSERT_EQ(compaction_reason, CompactionReason::kUniversalSizeRatio);
   }
 
   options.level0_file_num_compaction_trigger = 8;