ipv6: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ah6.c

@@ -36,7 +36,7 @@ struct tmp_ext {
 		struct in6_addr saddr;
 #endif
 		struct in6_addr daddr;
-		char hdrs[0];
+		char hdrs[];
 };
 
 struct ah_skb_cb {

net/ipv6/seg6_iptunnel.c

@@ -29,7 +29,7 @@
 
 struct seg6_lwt {
 	struct dst_cache cache;
-	struct seg6_iptunnel_encap tuninfo[0];
+	struct seg6_iptunnel_encap tuninfo[];
 };
 
 static inline struct seg6_lwt *seg6_lwt_lwtunnel(struct lwtunnel_state *lwt)