kernel/team: Actually fetch child thread times for B_TEAM_USAGE_CHILDREN.

Previously this code iterated over children, but only ever looked
at the parent team's thread times.

It seems this has been broken since the code was originally
introduced in 2004.

src/system/kernel/team.cpp

@@ -2804,7 +2804,7 @@ common_get_team_usage_info(team_id id, int32 who, team_usage_info* info,
 			for (; child != NULL; child = team->children.GetNext(child)) {
 				TeamLocker childLocker(child);
 
-				Thread* thread = team->thread_list;
+				Thread* thread = child->thread_list;
 
 				for (; thread != NULL; thread = thread->team_next) {
 					InterruptsSpinLocker threadTimeLocker(thread->time_lock);