gnop: change the "count until fail" option

Change the "count_until_fail" option of gnop, now it enables the failing
rating instead of setting them to 100%.

The original patch introduced the new flag, which sets the fail/rate to 100%
after N requests. In some cases, we don't want to have 100% of failure
probabilities. We want to start failing at some point.
For example, on the early stage, we may like to allow some read/writes requests
before having some requests delayed - when we try to mount the partition,
or when we are trying to import the pool.
Another case may be to check how scrub in ZFS will behave on different stages.

This allows us to cover more cases.
The previous behavior still may be configured.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D22632