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/proto/server.h b/include/proto/server.h
index 06f553e..ca794ab 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -261,7 +261,7 @@
 			_HA_ATOMIC_SUB(&srv->curr_idle_conns, 1);
 			return 0;
 		}
-		LIST_DEL_INIT(&conn->list);
+		MT_LIST_DEL(&conn->list);
 		MT_LIST_ADDQ(is_safe ? &srv->safe_conns[tid] : &srv->idle_conns[tid],
 		             (struct mt_list *)&conn->list);
 		srv->curr_idle_thr[tid]++;