net: constify sk_dst_get() and __sk_dst_get() argument

Both helpers only read fields from their socket argument.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -2141,14 +2141,14 @@ static inline bool sk_rethink_txhash(struct sock *sk)
 }
 
 static inline struct dst_entry *
-__sk_dst_get(struct sock *sk)
+__sk_dst_get(const struct sock *sk)
 {
 	return rcu_dereference_check(sk->sk_dst_cache,
 				     lockdep_sock_is_held(sk));
 }
 
 static inline struct dst_entry *
-sk_dst_get(struct sock *sk)
+sk_dst_get(const struct sock *sk)
 {
 	struct dst_entry *dst;