base-files: upgrade: take down loop and LVM before upgrade

base-files: upgrade: take down loop and LVM before upgrade

Users of devices with large block storage may choose to have an LVM
partition on the same device which is used for booting OpenWrt.
The presents a problem during sysupgrade as the root device is then
still busy and changing partitions will not work as desired,
leading to data corruption in case the newly flashed image is larger
than the currently installed one.
Having loop devices setup causes similar havoc.
Make sure all volume groups are offline and all loop devices have been
released before sysupgrade.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>