device_cgroup: Fix RCU list debugging warning

exceptions may be traversed using list_for_each_entry_rcu()
outside of an RCU read side critical section BUT under the
protection of decgroup_mutex. Hence add the corresponding
lockdep expression to fix the following false-positive
warning:

[    2.304417] =============================
[    2.304418] WARNING: suspicious RCU usage
[    2.304420] 5.5.4-stable Joshua-Riek#17 Tainted: G            E
[    2.304422] -----------------------------
[    2.304424] security/device_cgroup.c:355 RCU-list traversed in non-reader section!!

Signed-off-by: Amol Grover <[email protected]>
Signed-off-by: James Morris <[email protected]>

Author: frextrite

Diff for: security/device_cgroup.c

@@ -354,7 +354,8 @@ static bool match_exception_partial(struct list_head *exceptions, short type,
 {
 	struct dev_exception_item *ex;
 
-	list_for_each_entry_rcu(ex, exceptions, list) {
+	list_for_each_entry_rcu(ex, exceptions, list,
+				lockdep_is_held(&devcgroup_mutex)) {
 		if ((type & DEVCG_DEV_BLOCK) && !(ex->type & DEVCG_DEV_BLOCK))
 			continue;
 		if ((type & DEVCG_DEV_CHAR) && !(ex->type & DEVCG_DEV_CHAR))