zebra: use zserv_send_message instead of writen

zebra: use zserv_send_message instead of writen

Following functions is using writen to dispatch message
into socket, but another function uses zserv_send_message.
This commit does tiny unification for zapi's socket messaging.

Funcs:
- zsend_assign_label_chunk_response()
- zsend_label_manager_connect_response()

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>