sched: Fix DEBUGASSERT() in sched_unlock() for SMP

Summary:
- I noticed DEBUGASSERT() happens in sched_unlock()
- The test was Wi-Fi audio streaming stress test with spresense 3cores
- Actually, g_cpu_schedlock was locked but g_cpu_lockset was incorrect
- Finally, I found that cpu was obtained before enter_critical_section()
- And the task was moved from one cpu to another cpu
- However, that call should be done within the critical section
- This commit fixes this issue

Impact:
- Affects SMP only

Testing:
- Tested with spresense:wifi_smp (both NCPUS=2 and 3)
- Tested with lc823450-xgevk:rndis
- Tested with maix-bit:smp (QEMU)
- Tested with esp32-core:smp (QEMU)
- Tested with sabre-6quad:smp (QEMU)

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

sched/sched/sched_unlock.c

@@ -54,14 +54,12 @@
 int sched_unlock(void)
 {
   FAR struct tcb_s *rtcb;
-  int cpu;
 
   /* This operation is safe because the scheduler is locked and no context
    * switch may occur.
    */
 
-  cpu  = this_cpu();
-  rtcb = current_task(cpu);
+  rtcb = this_task();
 
   /* Check for some special cases:  (1) rtcb may be NULL only during
    * early boot-up phases, and (2) sched_unlock() should have no
@@ -73,6 +71,7 @@ int sched_unlock(void)
       /* Prevent context switches throughout the following. */
 
       irqstate_t flags = enter_critical_section();
+      int cpu = this_cpu();
 
       /* Decrement the preemption lock counter */