net: sctp: Fix IPv6 ancestor_size calc in sctp_copy_descendant

When calculating ancestor_size with IPv6 enabled, simply using
sizeof(struct ipv6_pinfo) doesn't account for extra bytes needed for
alignment in the struct sctp6_sock. On x86, there aren't any extra
bytes, but on ARM the ipv6_pinfo structure is aligned on an 8-byte
boundary so there were 4 pad bytes that were omitted from the
ancestor_size calculation.  This would lead to corruption of the
pd_lobby pointers, causing an oops when trying to free the sctp
structure on socket close.

Signed-off-by: Henry Ptasinski <hptasinski@google.com>

net/sctp/socket.c

@@ -9220,12 +9220,14 @@ void sctp_copy_sock(struct sock *newsk, struct sock *sk,
 static inline void sctp_copy_descendant(struct sock *sk_to,
 					const struct sock *sk_from)
 {
-	int ancestor_size = sizeof(struct inet_sock) +
-			    sizeof(struct sctp_sock) -
-			    offsetof(struct sctp_sock, pd_lobby);
+	size_t ancestor_size = sizeof(struct inet_sock);
 
 	if (sk_from->sk_family == PF_INET6)
-		ancestor_size += sizeof(struct ipv6_pinfo);
+		ancestor_size += sizeof(struct sctp6_sock);
+	else
+		ancestor_size += sizeof(struct sctp_sock);
+
+	ancestor_size -= offsetof(struct sctp_sock, pd_lobby);
 
 	__inet_sk_copy_descendant(sk_to, sk_from, ancestor_size);
 }