Iterator::Prev very slow

It took more than more than 360s to traverse 15GB (7342 files) data in reverse with `Prev`.

This is my logic:
```cpp
open db;
seek to a key;
NewIterator();
Prev(); // this call is very very slow.
```

I traced the problem with `strace`. Then I found that 'pread64' was executing most of time, about 320s. I also found more than 100 SSTable files were read.

I was confused why so many SSTable files were read. I guess that is the cause. So I got SSTable info with 'GetProperty(leveldb.sstables)' and try to find why so many files were read.
According to the code, I found at most 17 files were read which is much smaller than it actually was.

My version is 1.18-5.

Who has the same problem as me? Any suggestions are welcome.