fs-tests: integck: shrink write_info even more

fs-tests: integck: shrink write_info even more

The 'struct write_info' data strucutre records information about
truncation, and has a separate integer field to specify if this is
a truncation or not. But it is too wasteful because we have huge
amount of these objects.

But it is easy to notice that the 'random_seed' field is not used
for truncation records, so we can use that as the truncation flag.

This patch introduces MAX_RANDOM_SEED macro and when the 'random_seed'
field is greater than that, we treat it as the truncation flag.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>