Failure when booting after BTRFS snapshot rollback (Kernel panic)

[74cmonty]

Hi,
I have identified this issue on Debian 9.6 with linux-kernel 4.18.0,
snapper 0.8.2-1 and grub-common 2.02+dfsg1-8:
The system will not boot a snapshot that was rolled back with snapper
and run into Kernel panic when selecting default grub entry.

Steps to reproduce the issue:

 1. Boot snapshot X
 2. After system is loaded it is read-only and not fully usable
 3. Rollback snapshot X with command snapper rollback X
 4. snapper rollback will create additional snapshots, of which one is a
    read-write snapshot Y of snapshot X; in addition this snapshot Y
    will be set as default snapshot (corresponding to btrfs subvolume
    set-default Y)
 5. reboot and select default entry

A workaround to boot w/o errors is to edit the default grub enty by
means of deleting boot option rootflags=subvol=@ as snapper sets a new
default subvolume that is booted only w/o this rootflag option.

To make this modification available in Grub you must edit file
/etc/grub.d/10_linux
Replace line 79
GRUB_CMDLINE_LINUX="rootflags=subvol=${rootsubvol} ${GRUB_CMDLINE_LINUX}"
by
GRUB_CMDLINE_LINUX="${GRUB_CMDLINE_LINUX}"

and run "update-grub".