timers/migration: Simplify top level detection on group setup

Having a single group on a given level is enough to know this is the
top level, because a root has to have at least two children, unless that
root is the only group and the children are actual CPUs.

Simplify the test in tmigr_setup_groups() accordingly.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250114231507.21672-5-frederic@kernel.org

Author: Frederic Weisbecker

kernel/time/timer_migration.c

@@ -1624,9 +1624,7 @@ static int tmigr_setup_groups(unsigned int cpu, unsigned int node)
 		 * be different from tmigr_hierarchy_levels, contains only a
 		 * single group.
 		 */
-		if (group->parent || i == tmigr_hierarchy_levels ||
-		    (list_empty(&tmigr_level_list[i]) &&
-		     list_is_singular(&tmigr_level_list[i - 1])))
+		if (group->parent || list_is_singular(&tmigr_level_list[i - 1]))
 			break;
 
 	} while (i < tmigr_hierarchy_levels);