test/hash: fix off-by-one check on core count

This subtest wants to start rwc_core_cnt[n] reader threads, while the
master core is waiting for them to report.

Fixes: c7eb0972e74b ("test/hash: add lock-free r/w concurrency")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Acked-by: Yipeng Wang <yipeng1.wang@intel.com>