buf_flush_LRU_recommendation() is too optimistic

[Yasufumi Kinoshita]

buf_flush_LRU_recommendation() is counted without LRU_mutex, currently.
If buf_flush_LRU_recommendation() returns some non-zero value, buf_flush_batch(BUF_FLUSH_LRU.. is called using the value.

So, buf_flush_LRU_recommendation() should not return wrong value for optimistic performance.

buf_flush_LRU_recommendation() should count blocks again with LRU_mutex obtaining, if the return value may not be zero.