[$] XArray and the mainline

The XArray data structure was the topic ofthe final filesystem track session at the 2018 Linux Storage, Filesystem,and Memory-Management Summit (LSFMM). XArray is a new API for the kernel'sradix-tree data structure; the session wasled by Matthew Wilcox, who created XArray. When asked by Dave Chinner ifthe session was intended to be a live review of the patches, Wilcoxadmitted with a grin that it might be "the only way to get a review on thisdamn patch set".