After a snapper boot rollback, the default btrfs subvolume has changed to the
snapshot I've rolled back to.  This had several undesirable consequences:

1. Snapper stopped working since /.snapshots wasn't populated any more.  I've
fixed that by adding the .snapshots subvolume to /etc/fstab.

2. One of the snapshots listed by snapper is actually the new default
subvolume, but that's not marked in any way.

3. Trying to delete that snapshot leaves the system in an unusable state with
most filesystems unmounted.  You can only switch off hard at that point.  The
snapper cleanup hasn't gotten to that snapshot yet, but when it does it might
produce the same crash then.