buf_flush_LRU_tail(): incorrect buf_flush_LRU() return value check and srv_cleaner_eviction_factor handling

In buf_flush_LRU_tail():
- there is an incorrect else branch to if (buf_flush_LRU(...)). The code there should be executed unconditionally or the last flush batch is failed to be accounted.
- the (srv_cleaner_eviction_factor ? n.evicted : n.flushed) condition in instance stop check is inverted.