[$] configfd() and shifting bind mounts

The 5.2 kernel saw the addition of an extensive new API for the mounting(and remounting) of filesystems; thisarticle covered an early version of that API. Since then, work in thisarea has mostly focused on enabling filesystems to support this API fully.James Bottomley has taken a look at this API as part of the job ofredesigning his shiftfs filesystem andfound it to be incomplete. What has followed is a significant set ofchanges that promise to simplify the mount API - though it turns out that"simple" is often in the eye of the beholder.