Fix bug in DM iteration and standardise how to iterate through DM layers (#40426)

get_block_device_harder_fd() currently only traverses one level of
device mapper stacking when looking for the underlying block device.
This causes issues with nested DM setups like dm-crypt on top of
dm-integrity, where we don't traverse enough to get the actual physical
device.

Fix this by iterating through all DM layers until we reach a device with
no underlying device. And while we're at it also make cgroups use the
same logic.

Fixes: #40419

diff --cc src/repart/repart.c
Simple merge

diff --cc src/storagetm/storagetm.c
Simple merge