udp: preserve const qualifier in udp_sk()

We can change udp_sk() to propagate const qualifier of its argument,
thanks to container_of_const()

This should avoid some potential errors caused by accidental
(const -> not_const) promotion.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/udp.h

@@ -97,10 +97,7 @@ struct udp_sock {
 
 #define UDP_MAX_SEGMENTS	(1 << 6UL)
 
-static inline struct udp_sock *udp_sk(const struct sock *sk)
-{
-	return (struct udp_sock *)sk;
-}
+#define udp_sk(ptr) container_of_const(ptr, struct udp_sock, inet.sk)
 
 static inline void udp_set_no_check6_tx(struct sock *sk, bool val)
 {