net: lwm2m: Fix data cache rollback logic compilation

The root cause of this issue is a modification of `struct ring_buf` in
3075a7d9. Even though all the fields of the struct are marked as
internal, the LwM2M code is using some of them to roll back the state of
the ring buffer on failure.

Signed-off-by: Etienne de Maricourt <edmecomemail@gmail.com>

subsys/net/lib/lwm2m/lwm2m_message_handling.c

@@ -1403,9 +1403,9 @@ static int lwm2m_read_cached_data(struct lwm2m_message *msg,
 		read_info = &msg->cache_info->read_info[msg->cache_info->entry_size];
 		/* Store original timeseries ring buffer get states for failure handling */
 		read_info->cache_data = cached_data;
-		read_info->original_get_base = cached_data->rb.get_base;
-		read_info->original_get_head = cached_data->rb.get_head;
-		read_info->original_get_tail = cached_data->rb.get_tail;
+		read_info->original_get_base = cached_data->rb.get.base;
+		read_info->original_get_head = cached_data->rb.get.head;
+		read_info->original_get_tail = cached_data->rb.get.tail;
 		msg->cache_info->entry_size++;
 		if (msg->cache_info->entry_limit) {
 			length = MIN(length, msg->cache_info->entry_limit);
@@ -3079,11 +3079,11 @@ static bool lwm2m_timeseries_data_rebuild(struct lwm2m_message *msg, int error_c
 
 	/* Put Ring buffer back to original */
 	for (int i = 0; i < cache_temp->entry_size; i++) {
-		cache_temp->read_info[i].cache_data->rb.get_head =
+		cache_temp->read_info[i].cache_data->rb.get.head =
 			cache_temp->read_info[i].original_get_head;
-		cache_temp->read_info[i].cache_data->rb.get_tail =
+		cache_temp->read_info[i].cache_data->rb.get.tail =
 			cache_temp->read_info[i].original_get_tail;
-		cache_temp->read_info[i].cache_data->rb.get_base =
+		cache_temp->read_info[i].cache_data->rb.get.base =
 			cache_temp->read_info[i].original_get_base;
 	}