btreplay: Machines are now large enough that holes need to be dealt with

The current method fails if once we hit the first offlined cpu.  This
will correct that case.  However this still underreports the number cpus if
the last cpu are offlined.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Nathan Zimmer <nzimmer@sgi.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>