[UDP]: Allow users to configure UDP-Lite.

Let's give users an option for disabling UDP-Lite (~4K).

old:
|    text	   data	    bss	    dec	    hex	filename
|  286498	  12432	   6072	 305002	  4a76a	net/ipv4/built-in.o
|  193830	   8192	   3204	 205226	  321aa	net/ipv6/ipv6.o

new (without UDP-Lite):
|    text	   data	    bss	    dec	    hex	filename
|  284086	  12136	   5432	 301654	  49a56	net/ipv4/built-in.o
|  191835	   7832	   3076	 202743	  317f7	net/ipv6/ipv6.o

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
diff --git a/include/net/udplite.h b/include/net/udplite.h
index b76b2e3..01ddb2c 100644
--- a/include/net/udplite.h
+++ b/include/net/udplite.h
@@ -25,7 +25,9 @@
 /* Designate sk as UDP-Lite socket */
 static inline int udplite_sk_init(struct sock *sk)
 {
+#ifdef CONFIG_IP_UDPLITE
 	udp_sk(sk)->pcflag = UDPLITE_BIT;
+#endif
 	return 0;
 }
 
@@ -69,7 +71,7 @@
 static inline int udplite_sender_cscov(struct udp_sock *up, struct udphdr *uh)
 {
 	int cscov = up->len;
-
+#ifdef CONFIG_IP_UDPLITE
 	/*
 	 * Sender has set `partial coverage' option on UDP-Lite socket
 	 */
@@ -93,13 +95,15 @@
 	 *       illegal, we fall back to the defaults here.
 	 */
 	}
+#endif
 	return cscov;
 }
 
 static inline __wsum udplite_csum_outgoing(struct sock *sk, struct sk_buff *skb)
 {
-	int cscov = udplite_sender_cscov(udp_sk(sk), udp_hdr(skb));
 	__wsum csum = 0;
+#ifdef CONFIG_IP_UDPLITE
+	int cscov = udplite_sender_cscov(udp_sk(sk), udp_hdr(skb));
 
 	skb->ip_summed = CHECKSUM_NONE;     /* no HW support for checksumming */
 
@@ -112,6 +116,7 @@
 		if ((cscov -= len) <= 0)
 			break;
 	}
+#endif
 	return csum;
 }