UML/hostfs - mount failure at tip of tree

[Maciej Żenczykowski]

Reverting the following 3 patches:
- 104eef133fd9 hostfs: Add const qualifier to host_root in hostfs_fill_super()
- cd140ce9f611 hostfs: convert hostfs to use the new mount API
- e3ec0fe944d2 hostfs: Convert hostfs_read_folio() to use a folio

appears to be necessary to get the Android net test framework to boot
with tip of tree,
*without* the reverts we get:
  mount: /host: special device hostfs does not exist.
(if I don't revert the folio change then it mounts, but appears to not
actually work)

This is likely related to having an old debian mount binary...
but it is not *that* old... the base OS image was built in October 2022.

root@uml-x86-64:/# mount --version
mount from util-linux 2.36.1 (libmount 2.36.1: selinux, smack, btrfs,
namespaces, assert, debug)
root@uml-x86-64:/# dpkg -l | egrep mount
ii  libmount1:amd64               2.36.1-8+deb11u1             amd64
     device mounting library
ii  mount                         2.36.1-8+deb11u1             amd64
     tools for mounting and manipulating filesystems
which is from https://packages.debian.org/bullseye/mount [oldstable]

https://www.debian.org/releases/bullseye/
The Debian 11 life cycle encompasses five years: the initial three
years of full Debian support, until August 14th, 2024, and two years
of Long Term Support (LTS), until August 31st, 2026.

Thoughts?

--
Maciej Żenczykowski, Kernel Networking Developer @ Google