smc: preserve const qualifier in smc_sk()

We can change smc_sk() to propagate its argument const qualifier,
thanks to container_of_const().

Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Karsten Graul <[e-mail]>
Cc: Wenjia Zhang <[e-mail]>
Cc: Jan Karcher <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Reviewed-by: Wenjia Zhang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/smc/smc.h

···
 						 * */
 };
 
-static inline struct smc_sock *smc_sk(const struct sock *sk)
-{
-	return (struct smc_sock *)sk;
-}
+#define smc_sk(ptr) container_of_const(ptr, struct smc_sock, sk)
 
 static inline void smc_init_saved_callbacks(struct smc_sock *smc)
 {