client: partial revert of 9899aff3e345

client: partial revert of 9899aff3e345

We do need the client->nr_stat, otherwise things break when
group reporting is not used. We need it to keep track of
how many thread_stat's we are expecting to receive from
each client.

Signed-off-by: Jens Axboe <axboe@fb.com>