Debian/Ubuntu: Create /mnt/run/lock

The fix for #97, 66233a1138, seems to have
created another issue.

Closes #100

Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS.rst

@@ -698,6 +698,7 @@ Step 4: System Configuration
      mount --rbind /proc /mnt/proc
      mount --rbind /sys  /mnt/sys
      mount -t tmpfs tmpfs /mnt/run
+     mkdir /mnt/run/lock
      chroot /mnt /usr/bin/env DISK=$DISK UUID=$UUID bash --login
 
    **Note:** This is using ``--rbind``, not ``--bind``.