nd6: fix copying more than one DNS server

See bug #55163

(cherry picked from commit dcb29c591fdac0f073c4206e60e1fb94a577e0b8)

src/core/ipv6/nd6.c

@@ -766,7 +766,7 @@ nd6_input(struct pbuf *p, struct netif *inp)
 
         rdnss_opt = (struct rdnss_option *)buffer;
         num = (rdnss_opt->length - 1) / 2;
-        for (n = 0; (rdnss_server_idx < DNS_MAX_SERVERS) && (n < num); n++) {
+        for (n = 0; (rdnss_server_idx < DNS_MAX_SERVERS) && (n < num); n++, copy_offset += sizeof(ip6_addr_p_t)) {
           ip_addr_t rdnss_address;
 
           /* Copy directly from pbuf to get an aligned, zoned copy of the prefix. */