[enhancement](memtable) merge redundant mutex locks in _flush_memtable_async

The two consecutive lock_guard blocks were locking the same mutex
(_mem_table_ptr_lock) with no operations in between, which is redundant.

This commit merges them into a single lock scope to:
1. Reduce unnecessary lock/unlock overhead
2. Improve code readability

Author: carolinchen-docker

be/src/olap/memtable_writer.cpp

@@ -160,9 +160,6 @@ Status MemTableWriter::_flush_memtable_async() {
         std::lock_guard<std::mutex> l(_mem_table_ptr_lock);
         memtable = _mem_table;
         _mem_table = nullptr;
-    }
-    {
-        std::lock_guard<std::mutex> l(_mem_table_ptr_lock);
         memtable->update_mem_type(MemType::WRITE_FINISHED);
         _freezed_mem_tables.push_back(memtable);
     }