iotune: don't crash in sequential read test if hitting EOF
iotune: don't crash in sequential read test if hitting EOF

The sequential read test can easily hit EOF. It reads from the
files generated by the write bandwidth test, and two effects can
contribute to hitting EOF:

 1. read bandwidth may be greater than write bandwidth, so if
    the tests are set to run for the same amount of time, we'll
    hit EOF.
 2. the write test may run for a shorter time than the read
    test. Even though we allocate 70% of the test time to
    sequential writes, and 10% of the test time to sequential
    reads, those 70% are further divided per-shard (each shard
    writes for 1/smp::count of the total allocated time, to a
    different file). With many shards, the amount of data written
    by shard 0 drops dramatically.

Fix by wrapping around to offset 0. Since it happens very rarely,
the sequential nature of the test is preserved.

Ref scylladb/scylla#5181.
Message-Id: <c3db6314f2275350f2ef4afadedc7638bd6b8ad2@scylladb.com>

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