Bug#1018894: rescue-mode: mounts wrong btrfs subvolume

[Ansgar <ansgar@debian.org>]

Source: rescue
Version: 1.85
Severity: important

I've installed a system using btrfs for the root filesystem with d-i
(with disk encryption as well). As grub wasn't properly installed
(not registered with EFI), I tried to use the rescue mode to reinstall
grub.

However, mounting the root filesystem failed: /target contained only a
"@rootfs" subdirectory. So running a shell in the target fs failed.
Manually mounting the filesystem with `-o subvol=@rootfs` worked.

This was with Debian 11.4.

Ansgar