[$] DAX semantics

<p>In the filesystems track at the 2019 Linux Storage, Filesystem, andMemory-Management Summit, Ted Ts'o led a discussion about an inode flag toindicate DAX files, which is meant to be applied to files that should bedirectly accessed without going through the page cache. XFS has such aflag, but ext4 and other filesystems do not. The semantics of what the flag would mean arenot clear to Ts'o (and probably others), so the intent of the discussionwas to try to nail those down.