kernel: Optimize k_mem_slab_free()

Optimizes k_mem_slab_free() for the case where the slab's memory is
not exhausted.

Signed-off-by: Peter Mitsis <[email protected]>

Author: peter-mitsis

kernel/mem_slab.c

@@ -278,7 +278,7 @@ void k_mem_slab_free(struct k_mem_slab *slab, void *mem)
 	k_spinlock_key_t key = k_spin_lock(&slab->lock);
 
 	SYS_PORT_TRACING_OBJ_FUNC_ENTER(k_mem_slab, free, slab);
-	if ((slab->free_list == NULL) && IS_ENABLED(CONFIG_MULTITHREADING)) {
+	if (unlikely(slab->free_list == NULL) && IS_ENABLED(CONFIG_MULTITHREADING)) {
 		struct k_thread *pending_thread = z_unpend_first_thread(&slab->wait_q);
 
 		if (unlikely(pending_thread != NULL)) {