[RocksDB] Universal compaction trigger condition minor fix

Summary: Currently, when total number of files reaches level0_file_num_compaction_trigger, universal compaction will schedule a compaction job, but the job will not honor the compaction until the total number of files is level0_file_num_compaction_trigger+1. Fixed the condition for consistent behavior (start compaction on reaching level0_file_num_compaction_trigger).

Test Plan: make check; db_stress

Reviewers: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12945

db/version_set.cc

@@ -2367,7 +2367,7 @@ Compaction* VersionSet::PickCompactionUniversalReadAmp(
 Compaction* VersionSet::PickCompactionUniversal(int level, double score) {
   assert (level == 0);
 
-  if ((current_->files_[level].size() <=
+  if ((current_->files_[level].size() <
       (unsigned int)options_->level0_file_num_compaction_trigger)) {
     Log(options_->info_log, "Universal: nothing to do\n");
     return nullptr;