keybase client causing `/` mount issues under Ubuntu 22.04, Fedora 34 and newer

With the new version of the keybase ( `5.9.0-20211217212642.29bfd9d39f` ) that @heronhaye released last week, it's causing a very worrisome issue (which I initially thought to be a hard drive problem).

I've observed it now on two different unrelated Ubuntu 18.04 systems. Initially I didn't realize it was keybase until it happened on the second system and I thought it might be mount/fuse related.

# Symptoms

Intermittently the `/` mount in the OS stops working. Running `ls /` hangs and doesn't return. Launching nautilus hangs and doesn't launch. Other applications (in my case a java app called SmartGit) that try to read from `/` also hang. Other mounts appear ok, for example `ls /boot` works fine. Apps which don't do anything in the `/` mount (for example my browser which I guess works entirely in my home directory, and home is a distinct mount, works fine)

# Possible Solution

Some clients have seen this problem go away in the [v5.9.3](https://github.com/keybase/client/releases/tag/v5.9.3) release which you can update to via the normal means : https://keybase.io/docs/the_app/install_linux

Others are still seeing this same problem. I migrated away from using keybase entirely (now using Syncthing) due to this problem so I can't confirm if it's been fixed or not.

# Workaround

When this happened on the second Ubuntu machine and I realized it might be keybase, I ran `keybase ctl stop` and immediately `/` became available, nautilus would launch, everything was fixed.

I was able to discover the previous version number at https://prerelease.keybase.io/linux_binaries/deb/index.html

Then either

```
sudo apt install keybase=5.8.1-20210930160723.fefa22edc1
```
or fetch the deb package listed on https://prerelease.keybase.io/linux_binaries/deb/index.html and install it

```
wget https://s3.amazonaws.com/prerelease.keybase.io/linux_binaries/deb/keybase_5.8.1-20210930160723.fefa22edc1_amd64.deb
sudo dpkg -i keybase_5.8.1-20210930160723.fefa22edc1_amd64.deb
```

Then run this to tell `apt` to hold and not upgrade keybase to `5.9.0`

```
sudo apt-mark hold keybase
```

Once this issue is fixed you can unhold keybase with

```
sudo apt-mark unhold keybase
```

**MAKE SURE NOT TO FORGET THAT YOU PUT KEYBASE ON HOLD AS OTHERWISE IT WILL NEVER GET UPDATES AGAIN**

Start by reproducing the reported root-mount hangs with the affected Keybase versions on Ubuntu or Fedora, using the listed `keybase ctl stop` workaround as a comparison. Done means identifying the cause of the mount regression, addressing it, and verifying that `/` remains responsive while Keybase is running.

Written by the indexing model from the issue text.