thread cpu resource statistics bug fix

thread cpu resource statistics bug fix

If `thread' option is enabled, resource usage should be thread
based instead of process based. For the following job,

fio --ioengine=cpuio --cpuload=50 --time_based --runtime=10 --name=j0 --numjobs=4 --thread

before patch, each thread CPU statistics:
...
  cpu          : usr=199.67%, sys=0.14%, ctx=1475, majf=0, minf=24
...

after patch:
...
  cpu          : usr=49.80%, sys=0.00%, ctx=79, majf=0, minf=18446744073709538943
...

Signed-off-by: Jens Axboe <axboe@kernel.dk>