qa/workunits/rbd: make continuous export-diff test actually work

qa/workunits/rbd: make continuous export-diff test actually work

The current version is pretty useless:

- "rbd bench" writes the same byte (0xff) over and over again, so
  almost all checksumming is in vain
- snapshots are taken in a steady state (i.e. not under I/O), so no
  race conditions can get exposed
- even with these caveats, it's not wired up into the suite

Redo this workunit to be a reliable reproducer for the issue fixed
in the previous commit and wire it up for both krbd and rbd-nbd.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>