XArray and the mainline

I decided it was too late for the XArray code to make it into 4.18. I think on balance this was the right call; I've fixed three important (to me ;-) bugs since then -- two in the page cache conversion and one in the DAX conversion. I haven't found any bugs in the XArray implementation itself, but I did decide to change a couple of things in the interface.

I've asked for the XArray to be added to linux-next now that -rc1 is out:
https://lkml.kernel.org/r/20180617021521.GA18455@bombadil...