fix: Round-Robin pri under leveled compaction allows subcompactions b… (#12843)

Summary:
### Summary: Round-Robin pri under leveled compaction allows subcompactions by default is not compatible with PlainTable

```c++
bool Compaction::ShouldFormSubcompactions() const {
  if (cfd_ == nullptr) {
    return false;
  }

  // Round-Robin pri under leveled compaction allows subcompactions by default
  // and the number of subcompactions can be larger than max_subcompactions_
  if (cfd_->ioptions()->compaction_pri == kRoundRobin &&
      cfd_->ioptions()->compaction_style == kCompactionStyleLevel) {
    return output_level_ > 0;
  }

  if (max_subcompactions_ <= 1) {
    return false;
  }
```

PlainTable does not support Subcompaction, including when AdaptiveTable is applied to PlainTable.  subcompaction by default will result in the following error in some scenarios.

```c++
void PlainTableIterator::Seek(const Slice& target) {
  if (use_prefix_seek_ != !table_->IsTotalOrderMode()) {
    // This check is done here instead of NewIterator() to permit creating an
    // iterator with total_order_seek = true even if we won't be able to Seek()
    // it. This is needed for compaction: it creates iterator with
    // total_order_seek = true but usually never does Seek() on it,
    // only SeekToFirst().
    status_ = Status::InvalidArgument(
        "total_order_seek not implemented for PlainTable.");
    offset_ = next_offset_ = table_->file_info_.data_end_offset;
    return;
  }
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12843

Reviewed By: ajkr

Differential Revision: D59433477

Pulled By: cbi42

fbshipit-source-id: fb780ba7f7e8efdfedb7480abf14dd38e0b63677

db/compaction/compaction.cc

@@ -864,6 +864,11 @@ bool Compaction::ShouldFormSubcompactions() const {
     return false;
   }
 
+  if (cfd_->ioptions()->table_factory->Name() ==
+      TableFactory::kPlainTableName()) {
+    return false;
+  }
+
   // Round-Robin pri under leveled compaction allows subcompactions by default
   // and the number of subcompactions can be larger than max_subcompactions_
   if (cfd_->ioptions()->compaction_pri == kRoundRobin &&

db/compaction/compaction_job.cc

@@ -468,6 +468,11 @@ void CompactionJob::GenSubcompactionBoundaries() {
   ReadOptions read_options(Env::IOActivity::kCompaction);
   read_options.rate_limiter_priority = GetRateLimiterPriority();
   auto* c = compact_->compaction;
+  if (c->immutable_options()->table_factory->Name() ==
+      TableFactory::kPlainTableName()) {
+    return;
+  }
+
   if (c->max_subcompactions() <= 1 &&
       !(c->immutable_options()->compaction_pri == kRoundRobin &&
         c->immutable_options()->compaction_style == kCompactionStyleLevel)) {