librbd: fix DiffIterateStress test

If we write to an interval that didn't previously exist and then discard
it so that it again doesn't exist, all during the same interval, then we
should not include it in the 'written' set (or exists set, obviously).

Similarly, when we got to look at a merged diff, we can ignore extents
that were written (and possibly zeroed) if they neither existed before nor
after.

Bump up the iteration count to get more confidence that this is actually
correct.

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