This adds a fallback to `blockdev_get_root`, to handle the case where
`/usr` is a sysext overlay. Additinal changes are:
- move `extension_overlay_block` to `extension-util.h`, and make it
generic to work in both confext and sysext cases (though only sysext is
actually used currently)
- add a usr-only integration test to exercise the failure
Closes#43565.