options: flow should parse as FIO_OPT_INT

options: flow should parse as FIO_OPT_INT

It's an unsigned int, if we parse it as FIO_OPT_ULL then we clobber
the variable around it in the options struct. This is apparent when
running the examples/flow.fio job file, as flow_sleep=1000 is set
before flow, but gets clobbered when flow is set. This results in a
flow_sleep value of 0 instead of 1000.

Fixes: d4e74fda98b6 ("flow: add ability for weight-based flow control on multiple jobs")
Signed-off-by: Jens Axboe <axboe@kernel.dk>