sched: group: Fix group_signal() for SMP

Summary:
- This commit fixes the signest_test issue with lc823450-xgevk

Impact:
- SMP with CONFIG_SCHED_CHILD_STATUS=y

Testing:
- Tested with ostest with lc82345-xgevk:rndis
- NOTE: need to revert the commit 65274d5fdf

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

sched/group/group_signal.c

@@ -214,7 +214,11 @@ int group_signal(FAR struct task_group_s *group, FAR siginfo_t *siginfo)
    * nothing if were were called from an interrupt handler).
    */
 
+#ifdef CONFIG_SMP
+  irqstate_t flags = enter_critical_section();
+#else
   sched_lock();
+#endif
 
   /* Now visit each member of the group and perform signal handling checks. */
 
@@ -253,7 +257,11 @@ int group_signal(FAR struct task_group_s *group, FAR siginfo_t *siginfo)
     }
 
 errout:
+#ifdef CONFIG_SMP
+  leave_critical_section(flags);
+#else
   sched_unlock();
+#endif
   return ret;
 
 #else