shutdown fails to unmount some loops because of udevd, mark them LazyUnmount

[FAILED] Failed unmounting /run/mnt/data.
[FAILED] Failed unmounting /usr/lib/modules.
[ 461.816681] sd-umoun[2605]: Failed to unmount /oldroot: Device or resource busy
[ 461.826973] sd-remou[2606]: Failed to remount '/oldroot/run/mnt/data' read-only: Device or resource busy
[ 461.839423] sd-umoun[2607]: Failed to unmount /oldroot/run/mnt/data: Device or resource busy
[ 461.850739] sd-umoun[2608]: Failed to unmount /oldroot/run: Device or resource busy
[ 462.004953] shutdown[1]: Could not detach loopback /dev/loop0: Device or resource busy
[ 462.016063] shutdown[1]: Failed to finalize file systems, loop devices, ignoring
[ 462.049676] reboot: Power down

those messages are mostly harmless => systemd-udevd memorymaps modules.* files from the modules mount, and we don't stop udevd during shutdown (because, I don't know, it never was stopped), hence failure to unmount those things.