matrix: fix Product index offset and minscanlen value
matrix: fix Product index offset and minscanlen value

Incrementing the index on each iteration is problematic because you
may not see a particular element for 2*minscanlen indices.
Decrementing it ensures we'll see any particular submat element every
max(minscanlen) + 1 indicies.

Signed-off-by: Samuel Just <16ccadb3f2bf0703b4be974f8606e7e98c1cda29@redhat.com>

dmick/teuthology,dmick/teuthology,ceph/teuthology,dmick/teuthology,SUSE/teuthology,ceph/teuthology,ktdreyer/teuthology,SUSE/teuthology,SUSE/teuthology,ktdreyer/teuthology