IO latency/throughput fixes

Hi,

This is a set of patches that I worked on today in the hopes
of furthering the latency goals and at least fixing some of
the write regression with fwrite + fsync that current -git
is suffering from.

I haven't done any latency tests yet, I'm just tossing this
out there so we can collaborate on improving things. What I
did test was the silly fwrite() + fsync() loop test, which
is a LOT slower in current -git that it used to be. The test
is basically:

	while (nr--) {
		f = fopen();
		fprintf(f, "Some data here\n");
		fsync(fileno(f));
		fclose(f);
	}

which (for nr == 2000) takes 16 seconds in -git, completes
in 0.9s with the patches.

-- 
Jens Axboe

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