ceph_test_rados_api_tier: do fewer writes in HitSetWrite

We don't need to do quite so many writes.  It can be slow when we are
thrashing and aren't doing anything in parallel.

Fixes: #8932
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit c5f766bb16c0ab3c3554e73791ad0b74077ad35c)