base-files: upgrade: try umount lvm and loop devices

Try umount on device mapper and loop devices still mounted, so the
subsequent call to disactivate all physical volumes and delete all
loop devices is more likely to succeed.

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

package/base-files/files/lib/upgrade/stage2

@@ -64,6 +64,10 @@ switch_to_ramfs() {
 	/bin/mount -o remount,ro /mnt
 	/bin/umount -l /mnt
 
+	grep -e "^/dev/dm-.*" -e "^/dev/loop.*" /proc/mounts | while read bdev mp _r; do
+		umount $mp
+	done
+
 	[ "$RAMFS_COPY_LOSETUP" ] && losetup -D
 	[ "$RAMFS_COPY_LVM" ] && {
 		mkdir -p /tmp/lvm/cache