udev: when btrfs.ko is not available consider btrfs filesystems not ready

Let's add a special tweak to the btrfs builtin: if /dev/btrfs-control is
not there, let's consider all btrfs file systems as SYSTEMD_READY=0.
This is useful in initrds, where btrfs.ko might be missing. After the
initrd → host transition we can then retigger the device and undo the
SYSTEMD_READY=0 marking.