kernel: mem_slab: Optimize mem_slab_free

Changed algorithm of checking for pending thread in mem_slab.
Check for pending thread now is done only if there is
no memory left.

Signed-off-by: Kamil Lazowski <Kamil.Lazowski@nordicsemi.no>

kernel/mem_slab.c

@@ -143,16 +143,19 @@ int k_mem_slab_alloc(struct k_mem_slab *slab, void **mem, k_timeout_t timeout)
 void k_mem_slab_free(struct k_mem_slab *slab, void **mem)
 {
 	k_spinlock_key_t key = k_spin_lock(&lock);
-	struct k_thread *pending_thread = z_unpend_first_thread(&slab->wait_q);
 
-	if (pending_thread != NULL) {
-		z_thread_return_value_set_with_data(pending_thread, 0, *mem);
-		z_ready_thread(pending_thread);
-		z_reschedule(&lock, key);
-	} else {
-		**(char ***)mem = slab->free_list;
-		slab->free_list = *(char **)mem;
-		slab->num_used--;
-		k_spin_unlock(&lock, key);
+	if (slab->free_list == NULL) {
+		struct k_thread *pending_thread = z_unpend_first_thread(&slab->wait_q);
+
+		if (pending_thread != NULL) {
+			z_thread_return_value_set_with_data(pending_thread, 0, *mem);
+			z_ready_thread(pending_thread);
+			z_reschedule(&lock, key);
+			return;
+		}
 	}
+	**(char ***) mem = slab->free_list;
+	slab->free_list = *(char **) mem;
+	slab->num_used--;
+	k_spin_unlock(&lock, key);
 }