fix bug in printing eviction success rate

Summary:
This change fixes below bug printing wrong eviction success rate.

Evict Attempts: 439,372 Success: -4198434145486952.50%
RAM Evictions : 618,369

This happens because of the overflow in below code https://fburl.com/code/ln24ozv0

```
    out << folly::sformat(
               "Evict Attempts: {:,} Success: {:.2f}%",
               evictAttempts,
               invertPctFn(evictAttempts - numEvictions, evictAttempts)
```

Again, this could happen because evictAttempts and numEvictions are not taken atomically
https://fburl.com/code/i587dhfh

Reviewed By: therealgymmy

Differential Revision: D46043428

fbshipit-source-id: 022a07e09c1093ca705e9e57596fa912497ba32d

Author: Jaesoo Lee

cachelib/cachebench/cache/CacheStats.h

@@ -118,10 +118,9 @@ struct Stats {
                           allocAttempts,
                           invertPctFn(allocFailures, allocAttempts))
         << std::endl;
-    out << folly::sformat(
-               "Evict Attempts: {:,} Success: {:.2f}%",
-               evictAttempts,
-               invertPctFn(evictAttempts - numEvictions, evictAttempts))
+    out << folly::sformat("Evict Attempts: {:,} Success: {:.2f}%",
+                          evictAttempts,
+                          pctFn(numEvictions, evictAttempts))
         << std::endl;
     out << folly::sformat("RAM Evictions : {:,}", numEvictions) << std::endl;