rbd: "rbd bench" always writes the same byte

It's expected that the buffer is filled with the same byte, but the
byte should differ from run to run:

    memset(bp.c_str(), rand() & 0xff, io_size);

This was broken in commit c7f71d1 ("rbd: migrated existing command
logic to new namespaces") which inadvertently moved the call to srand(),
leaving rand() unseeded for the above memset().

Fixes: https://tracker.ceph.com/issues/67698
Signed-off-by: Ilya Dryomov <[email protected]>

Author: idryomov

src/tools/rbd/action/Bench.cc

@@ -233,6 +233,9 @@ int do_bench(librbd::Image& image, io_type_t io_type,
     return r;
   }
 
+  // seed rand() before constructing rbd_bencher
+  srand(time(NULL) % (unsigned long) -1);
+
   rbd_bencher b(&image, io_type, io_size);
 
   std::cout << "bench "
@@ -261,8 +264,6 @@ int do_bench(librbd::Image& image, io_type_t io_type,
   }
   std::cout << std::endl;
 
-  srand(time(NULL) % (unsigned long) -1);
-
   coarse_mono_time start = coarse_mono_clock::now();
   std::chrono::duration<double> last = std::chrono::duration<double>::zero();
   uint64_t ios = 0;