[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>

include/net/udp.h

@@ -48,7 +48,7 @@ extern struct proto udp_prot;
 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,

net/ipv4/udp.c

@@ -139,7 +139,7 @@ static inline int udp_lport_inuse(u16 num)
  *  @saddr_comp:  AF-dependent comparison of bound local IP addresses
  */
 int udp_get_port(struct sock *sk, unsigned short snum,
-		 int (*saddr_cmp)(struct sock *sk1, struct sock *sk2))
+		 int (*saddr_cmp)(const struct sock *sk1, const struct sock *sk2))
 {
 	struct hlist_node *node;
 	struct hlist_head *head;
@@ -210,7 +210,7 @@ fail:
 	return error;
 }
 
-static inline int  ipv4_rcv_saddr_equal(struct sock *sk1, struct sock *sk2)
+static inline int ipv4_rcv_saddr_equal(const struct sock *sk1, const struct sock *sk2)
 {
 	struct inet_sock *inet1 = inet_sk(sk1), *inet2 = inet_sk(sk2);