bdi write bandwidth estimation

Do bdi write bandwidth estimation in the flusher thread at 200ms intervals,
and in case the flusher is blocked syncing large files, the throttled dirtier
tasks will back it up.

To get an idea of the adaption speed and fluctuation range, here are
some real examples (check the red dots and the yellow line):

http://www.kernel.org/pub/linux/kernel/people/wfg/writeba...
http://www.kernel.org/pub/linux/kernel/people/wfg/writeba...
http://www.kernel.org/pub/linux/kernel/people/wfg/writeba...
http://www.kernel.org/pub/linux/kernel/people/wfg/writeba...

The old version outputs, for your reference:

http://www.kernel.org/pub/linux/kernel/people/wfg/writeba...
http://www.kernel.org/pub/linux/kernel/people/wfg/writeba...
http://www.kernel.org/pub/linux/kernel/people/wfg/writeba...

This is merely the estimation part. The in-kernel users of the estimated
bandwidth will follow in the coming series.

Thanks,
Fengguang

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