sched: pthread: Fix pthread_join() for SMP

Summary:
- I noticed 'pthread_rwlock test' in ostest sometimes stops
- This issue happened with spresense:wifi_smp (NCPUS=4) and sim:smp
- Finally, I found an issue in pthread_join()
- In pthread_join(), sched_lock() is used to avoid pre-emption
- However, this is not enough for SMP
- Because another CPU would continue the pthread and exit sequences
- So we need to protect it with a critical section

Impact:
- Affect SMP only

Testing:
- Tested with ostest with the following configurations
- spresnese:smp
- spresense:wifi_smp (NCPUS=2, NCPUS=4)
- sabre-6quad:smp (QEMU)
- esp32-core:smp (QEMU)
- maix-bit:smp (QEMU)
- sim:smp
- lc823450-xgevk:rndis

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

sched/pthread/pthread_join.c

@@ -137,6 +137,15 @@ int pthread_join(pthread_t thread, FAR pthread_addr_t *pexit_value)
     }
   else
     {
+      /* NOTE: sched_lock() is not enough for SMP
+       * because another CPU would continue the pthread and exit
+       * sequences so need to protect it with a critical section
+       */
+
+#ifdef CONFIG_SMP
+      irqstate_t flags = enter_critical_section();
+#endif
+
       /* We found the join info structure.  Increment for the reference
        * to the join structure that we have.  This will keep things
        * stable for we have to do
@@ -211,6 +220,10 @@ int pthread_join(pthread_t thread, FAR pthread_addr_t *pexit_value)
 
       sched_unlock();
 
+#ifdef CONFIG_SMP
+      leave_critical_section(flags);
+#endif
+
       /* Release our reference to the join structure and, if the reference
        * count decrements to zero, deallocate the join structure.
        */