bench: fix use of fd after close

benchmark/io.c

@@ -51,7 +51,6 @@ static void writetest(int size, size_t bsize)
       exit(254);
     }
   }
-  close(fd);
 
 #ifndef NSYNC
 # ifdef __linux__
@@ -61,6 +60,8 @@ static void writetest(int size, size_t bsize)
 # endif
 #endif /* SYNC */
 
+  close(fd);
+
   end = now();
   elapsed = (end - start) / 1e6;
   mbps = ((tsize/elapsed)) / 1048576;