c654ffe34b
After 546556b71b we can have mixed writes into commitlog,
some do flush immediately some do not. If non flushing write races with
flushing one and becomes responsible for writing back its buffer into a
file flush will be skipped which will cause assert in batch_cycle() to
trigger since flush position will not be advanced. Fix that by checking
that flush was skipped and in this case flush explicitly our file
position.
Fixes #5670
Message-Id: <20200128145103.GI26048@scylladb.com>