diagnostics: wilco_dtc: add parameter to rootdev to handle device mapper

BUG=b:135375304
TEST=emerge-sarien diagnostics && cros deploy dut diagnostics;
start wilco_dtc

Change-Id: Ie7537d060d652f4dd0cbab256e81a156875ce941
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1661767
Tested-by: Trent Begin <tbegin@chromium.org>
Auto-Submit: Trent Begin <tbegin@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Commit-Queue: Trent Begin <tbegin@chromium.org>
(cherry picked from commit 791a7f9c97781e221089fa65868a30c8f96acec7)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1663891
Reviewed-by: Trent Begin <tbegin@chromium.org>
diff --git a/diagnostics/init/wilco_dtc.conf b/diagnostics/init/wilco_dtc.conf
index fadf33b..5ef9a6e 100644
--- a/diagnostics/init/wilco_dtc.conf
+++ b/diagnostics/init/wilco_dtc.conf
@@ -77,7 +77,7 @@
 script
 
   # Give wilco_dtc permission to read the diagnostics parition
-  DIAGNOSTICS_PARTITION="$(cgpt find -l DIAGNOSTICS $(rootdev -d) | head -1)"
+  DIAGNOSTICS_PARTITION="$(cgpt find -l DIAGNOSTICS $(rootdev -d -s) | head -1)"
   setfacl -m u:wilco_dtc:r "${DIAGNOSTICS_PARTITION}"
 
   exec minijail0 -e -l -n -p -r -t -v --uts \