MINOR: connections: Make the "list" element a struct mt_list instead of list.

Make the "list" element a struct mt_list, and explicitely use
list_from_mt_list to get a struct list * where it is used as such, so that
mt_list_for_each_entry will be usable with it.
diff --git a/include/types/connection.h b/include/types/connection.h
index 0136be9..32a8590 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -458,7 +458,7 @@
 
 	/* second cache line */
 	struct wait_event *subs; /* Task to wake when awaited events are ready */
-	struct list list;             /* attach point to various connection lists (idle, ...) */
+	struct mt_list list;          /* attach point to various connection lists (idle, ...) */
 	struct list session_list;     /* List of attached connections to a session */
 	union conn_handle handle;     /* connection handle at the socket layer */
 	const struct netns_entry *proxy_netns;