writeback data integrity and other fixes (take 3)

OK, I'm happier with this patchset now. Note that I've taken your patch
and mangled it a bit at the end of the series.

This one survives and seems to run OK here, but I'm mainly doing dumb
stress testing with a handful of filesystems, and data-io error injection
testing. There are a lot of combinations of ways this function can operate
and interact obviously, so it would be helpful to get more review.

Chris, would you possibly have time to run your btrfs tests that are
sensitive to problems in this code? I could provide you a single patch
rollup against mainline if it helps.

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