sched/semaphore/spinlock.c: Add memory barrier operations in spin_unlock()

In ARM document regarding memory barrires, SP_DMB() must be issued
before changing a spinlock state to SP_UNLOCKED. However, we found
that SP_DSB() is also needed to ensure that spin_unlock() works
correctly for network streaming aging test.

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

sched/semaphore/spinlock.c

@@ -165,8 +165,9 @@ void spin_unlock(FAR volatile spinlock_t *lock)
   sched_note_spinunlock(this_task(), lock);
 #endif
 
-  *lock = SP_UNLOCKED;
   SP_DMB();
+  *lock = SP_UNLOCKED;
+  SP_DSB();
 }
 #endif