lfsr: Fix spin related bug

On the previous patch on cycle detection, we would incremet spin value
when:

    num_vals % cycle_length == 0

which would evaluate always to true for the first iteration since we
start with num_vals = 0. Only one corner-case is affected by this bug
and this patch should fix it.

Signed-off-by: Alex Pyrgiotis <apyrgio@grnet.gr>
Signed-off-by: Jens Axboe <axboe@kernel.dk>