dns: Add API to clear dns cache

Picked from espressif/esp-lwip@ee59f77d
Ref IDF-4839

src/core/dns.c

@@ -382,6 +382,15 @@ dns_setserver(u8_t numdns, const ip_addr_t *dnsserver)
   }
 }
 
+/**
+ * Clears the DNS cache
+ */
+void
+dns_clear_cache(void)
+{
+  memset(dns_table, 0, sizeof(struct dns_table_entry) * DNS_TABLE_SIZE);
+}
+
 /**
  * @ingroup dns
  * Obtain one of the currently configured DNS server.

src/include/lwip/dns.h

@@ -111,9 +111,7 @@ err_t            dns_gethostbyname(const char *hostname, ip_addr_t *addr,
 err_t            dns_gethostbyname_addrtype(const char *hostname, ip_addr_t *addr,
                                    dns_found_callback found, void *callback_arg,
                                    u8_t dns_addrtype);
-#if ESP_DNS
-void             dns_clear_servers(bool keep_fallback);
-#endif
+void             dns_clear_cache(void);
 
 #if DNS_LOCAL_HOSTLIST
 size_t         dns_local_iterate(dns_found_callback iterator_fn, void *iterator_arg);