net/tcp: refactor tcp_inet6_sk()

Don't keep hand coded offset caluclations and replace it with
container_of(). It should be type safer and a bit less confusing.

It also makes it with a macro instead of inline function to preserve
constness, which was previously casted out like in case of
tcp_v6_send_synack().

Signed-off-by: Pavel Begunkov <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/tcp_ipv6.c

···
  * This avoids a dereference and allow compiler optimizations.
  * It is a specialized version of inet6_sk_generic().
  */
-static struct ipv6_pinfo *tcp_inet6_sk(const struct sock *sk)
-{
-	unsigned int offset = sizeof(struct tcp6_sock) - sizeof(struct ipv6_pinfo);
-
-	return (struct ipv6_pinfo *)(((u8 *)sk) + offset);
-}
+#define tcp_inet6_sk(sk) (&container_of_const(tcp_sk(sk), \
+					      struct tcp6_sock, tcp)->inet6)
 
 static void inet6_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
 {
···
 			      struct sk_buff *syn_skb)
 {
 	struct inet_request_sock *ireq = inet_rsk(req);
-	struct ipv6_pinfo *np = tcp_inet6_sk(sk);
+	const struct ipv6_pinfo *np = tcp_inet6_sk(sk);
 	struct ipv6_txoptions *opt;
 	struct flowi6 *fl6 = &fl->u.ip6;
 	struct sk_buff *skb;