support Huawei DSVPN's NHRP implementation

If MTU in the request packet is 0, Huawei NHRP daemon will not reply any packet. Hard code MTU to 9216 (Cisco IOS XE 16 also use this value) is a valid workaround.

Tested on Huaweu USG6320, software version V500R005C00SPC200

nhrp/nhrp_peer.c

@@ -828,7 +828,7 @@ static void nhrp_peer_send_protocol_purge(struct nhrp_peer *peer)
 
 	*cie = (struct nhrp_cie) {
 		.hdr.code = NHRP_CODE_SUCCESS,
-		.hdr.mtu = 0,
+		.hdr.mtu = htons(9216),
 		.hdr.preference = 0,
 		.hdr.prefix_length = 0xff,
 	};
@@ -1336,7 +1336,7 @@ static void nhrp_peer_send_resolve(struct nhrp_peer *peer)
         *cie = (struct nhrp_cie) {
 		.hdr.code = NHRP_CODE_SUCCESS,
 		.hdr.prefix_length = 0,
-		.hdr.mtu = 0,
+		.hdr.mtu = htons(9216),
 		.hdr.holding_time = htons(peer->interface->holding_time),
 	};