[UDP]: saddr_cmp function should take const socket pointers

This also kills a warning while building ipv6:

net/ipv6/udp.c: In function ‘udp_v6_get_port’:
net/ipv6/udp.c:66: warning: passing argument 3 of ‘udp_get_port’ from incompatible pointer type

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/udp.h b/include/net/udp.h
index c490a0f..db0c05f 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -48,7 +48,7 @@
 struct sk_buff;
 
 extern int	udp_get_port(struct sock *sk, unsigned short snum,
-			     int (*saddr_cmp)(struct sock *, struct sock *));
+			     int (*saddr_cmp)(const struct sock *, const struct sock *));
 extern void	udp_err(struct sk_buff *, u32);
 
 extern int	udp_sendmsg(struct kiocb *iocb, struct sock *sk,