fs-tests: use independent random generators for ops and data

fs-tests: use independent random generators for ops and data

Currently integck uses the same global random generator for everything -
for choosing the operation, generating the data, and for checking. This
makes integck to become stuck sometimes. My guess this is because of
we somehow re-set it back with srand() when checking files.

This patch makes integck use different generators for data and for
choosing operations by using rand_r() with own seed for operations.

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