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/sysupdate/sysupdate-resource.c

@@ -678,7 +678,7 @@ static int get_sysext_overlay_block(const char *p, dev_t *ret) {
                 return 0;
         }
 
-        (void) block_get_originating(*ret, ret);
+        (void) block_get_originating(*ret, ret, /* recursive= */ false);
         return 1;
 }