Cast input argument for json_object_add_value_int to long long

The latency related (and some other) variables type is uint32_t
but the JSON integer type is long long. So we need to cast
the numbers to long long type to make JSON output correctly.

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Signed-off-by: Jens Axboe <axboe@fb.com>