MINOR: dns: Maximum DNS udp payload set to 8192

Following up DNS extension introduction, this patch aims at making the
computation of the maximum number of records in DNS response dynamic.
This computation is based on the announced payload size accepted by
HAProxy.
diff --git a/include/types/dns.h b/include/types/dns.h
index 0f9c1b9..c7338c7 100644
--- a/include/types/dns.h
+++ b/include/types/dns.h
@@ -30,7 +30,7 @@
  */
 #define DNS_MAX_LABEL_SIZE	63
 #define DNS_MAX_NAME_SIZE	255
-#define DNS_MAX_UDP_MESSAGE	512
+#define DNS_MAX_UDP_MESSAGE	8192
 
 /* DNS minimun record size: 1 char + 1 NULL + type + class */
 #define DNS_MIN_RECORD_SIZE	( 1 + 1 + 2 + 2 )