sched: group: Fix group_kill_children() for SMP

Summary:
- During testing iperf -s with lc823450-xgevk:rndis,
  I noticed that hard fault happens
- Finally, I found that group_kill_children() is not
  not protected by a critical section
- This commit fixes this issue

Impact:
- SMP only

Testing:
- Tested with iperf with the following configurations
- lc823450-xgevk:rndis, spresense:rndis_smp
- Tested with ostest with the following configurations
- lc823450-xgevk:rndis, esp32-devkitc:smp (QEMU)
- sabre-6quad:smp (QEMU), maix-bit:smp (QEMU)
- spresense:rndis_smp, sim:smp

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

sched/group/group_killchildren.c

@@ -131,14 +131,26 @@ int group_kill_children(FAR struct tcb_s *tcb)
 {
   int ret;
 
+#ifdef CONFIG_SMP
+  /* NOTE: sched_lock() is not enough for SMP
+   * because tcb->group will be accessed from the child tasks
+   */
+
+  irqstate_t flags = enter_critical_section();
+#else
   /* Lock the scheduler so that there this thread will not lose priority
    * until all of its children are suspended.
    */
 
   sched_lock();
+#endif
   ret = group_foreachchild(tcb->group, group_kill_children_handler,
                            (FAR void *)((uintptr_t)tcb->pid));
+#ifdef CONFIG_SMP
+  leave_critical_section(flags);
+#else
   sched_unlock();
+#endif
   return ret;
 }