Needless flush list mutex reacquisitions in adaptive flushing

Introduced by bug 1083058 fix, credit to Sergei.

One of adaptive flushing bits in srv_master_thread() reads

     mutex_exit(&flush_list_mutex);

     flushed_blocks_num = new_blocks_num + prev_flush_info.count
        - blocks_num;
     if (flushed_blocks_num < 0) {
      flushed_blocks_num = 0;
     }

     mutex_enter(&flush_list_mutex);
     bpage = UT_LIST_GET_FIRST(buf_pool->flush_list);

     prev_flush_info.count = UT_LIST_GET_LEN(buf_pool->flush_list);
     if (bpage) {
      prev_flush_info.space = bpage->space;
      prev_flush_info.offset = bpage->offset;
      prev_flush_info.oldest_modification = bpage->oldest_modification;
      mutex_exit(&flush_list_mutex);
     } else {
      mutex_exit(&flush_list_mutex);
      prev_flush_info.space = 0;
      prev_flush_info.offset = 0;
      prev_flush_info.oldest_modification = 0;
     }

It makes no sense to release the flush list mutex for such a short duration and then reacquire it for very short duration again.