[PATCH][next] net: inet_sock.h: Avoid thousands of -Wflex-array-member-not-at-end warnings

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

Use the new TRAILING_OVERLAP() helper to fix 2600 of the following
warnings:

2600 ./include/net/inet_sock.h:65:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

This helper creates a union between a flexible-array member (FAM)
and a set of members that would otherwise follow it (in this case
`char data[40];) This overlays the trailing members (data) onto the FAM
(__data) while keeping the FAM and the start of MEMBERS aligned.

The static_assert() ensures this alignment remains, and it's
intentionally placed inmediately after `struct ip_options_data`
(no blank line in between).

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---

I think it's worth mentioning that the introduction of the new
TRAILING_OVERLAP() helper saves us from making changes like the
following, for this particular case:

	https://lore.kernel.org/linux-hardening/ZzK-n_C2yl8mW2Tz@kspp/

Thanks

 include/net/inet_sock.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index 1086256549fa..a974588803af 100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h
@@ -62,9 +62,12 @@ struct ip_options_rcu {
 };
 
 struct ip_options_data {
-	struct ip_options_rcu	opt;
-	char			data[40];
+	TRAILING_OVERLAP(struct ip_options_rcu, opt, opt.__data,
+			 char			data[40];
+	);
 };
+static_assert(offsetof(struct ip_options_data, opt.opt.__data) ==
+	      offsetof(struct ip_options_data, data));
 
 struct inet_request_sock {
 	struct request_sock	req;
-- 
2.43.0