common: disable btrfs fsck in fsck_types

Btrfs doesn't need nor use a regular fsck utility. This was wrongly added in b31a5d9f and currently makes genfstab generate btrfs entries with fs_passno not set to 0, which was set in previous releases.

I'd like to keep the entry in fsck_types and explicitly set it to 0 for future-proof reasons.