net/udp: Fix some errors in conditional compilation.

net/udp/udp.h

@@ -115,7 +115,7 @@ struct udp_conn_s
   struct iob_queue_s readahead;   /* Read-ahead buffering */
 #endif
 
-#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
+#ifdef CONFIG_NET_UDP_WRITE_BUFFERS
   /* Write buffering
    *
    *   write_q   - The queue of unsent I/O buffers.  The head of this

net/udp/udp_psock_sendto_buffered.c

@@ -684,7 +684,9 @@ ssize_t psock_udp_sendto(FAR struct socket *psock, FAR const void *buf,
       /* Initialize the write buffer */
 
       memcpy(&wrb->wb_dest, to, tolen);
+#ifdef CONFIG_NET_SOCKOPTS
       wrb->wb_start = clock_systimer();
+#endif
 
       /* Copy the user data into the write buffer.  We cannot wait for
        * buffer space if the socket was opened non-blocking.