klog 1.130.0: race condition during periodic flushing and rotating files

[pohly]

/kind bug

What steps did you take and what happened:

#403 moved the Sync method call out of the protection of the l.mu mutex. However, that means that the access to sb.file is no longer protected inside that method.

What did you expect to happen:

Let's remove the flushSyncWriter interface. We can instead use io.Writer and when it comes to flushing, check directly for the syncBuffer type to flush and sync.

Also, avoid allocating a slice each time flushing is done.

Anything else you would like to add:

Found during review of kubernetes/kubernetes#125531.

[k8s-ci-robot]

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