kernel/mutex: Clean up k_mutex_unlock()

Recent changes to the scheduler API means we can simplify this
further: move the assignment to mutex->owner outside the if(), which
removes the need to have an else clause (which just set that field to
NULL when the new_owner was already NULL); and we can likewise move
the irq_unlock() outside the block.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

kernel/mutex.c

@@ -225,6 +225,8 @@ void _impl_k_mutex_unlock(struct k_mutex *mutex)
 
 	struct k_thread *new_owner = _unpend_first_thread(&mutex->wait_q);
 
+	mutex->owner = new_owner;
+
 	K_DEBUG("new owner of mutex %p: %p (prio: %d)\n",
 		mutex, new_owner, new_owner ? new_owner->base.prio : -1000);
 
@@ -241,14 +243,12 @@ void _impl_k_mutex_unlock(struct k_mutex *mutex)
 		 * waiter since the wait queue is priority-based: no need to
 		 * ajust its priority
 		 */
-		mutex->owner = new_owner;
 		mutex->lock_count++;
 		mutex->owner_orig_prio = new_owner->base.prio;
-	} else {
-		irq_unlock(key);
-		mutex->owner = NULL;
 	}
 
+	irq_unlock(key);
+
 	k_sched_unlock();
 }