iotune: let caller of write_sequential_data adjust duration
iotune: let caller of write_sequential_data adjust duration

Per-shard write_sequential_data does not know if it's running
in parallel to other shards or serially.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

dreamsxin/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar