librbd: fix DiffIterateStress again

- fix seed
- the array indices are points in time; no need to subtract one from i!
- pick a random seed and print it to stdout

I ran this with several different seeds without failure, so I am confident
we are in good shape.  And if we ever get a future failure, we'll have the
seed to reproduce.

Signed-off-by: Sage Weil <sage@inktank.com>