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

src/core/cgroup.c

@@ -973,8 +973,7 @@ static int lookup_block_device(const char *p, dev_t *ret) {
         }
 
         /* If this is a LUKS/DM device, recursively try to get the originating block device */
-        while (block_get_originating(*ret, ret) >= 0)
-                ;
+        (void) block_get_originating(*ret, ret, /* recursive= */ true);
 
         /* If this is a partition, try to get the originating block device */
         (void) block_get_whole_disk(*ret, ret);