rbd: "rbd bench" always writes the same byte

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 c7f71d14a5d3 ("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 <idryomov@gmail.com>
(cherry picked from commit 907e53aa8524971e3d969ec519953cdd3d83871f)