net: netlink: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

---
 net/netlink/af_netlink.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 4e31721e7293..bced11032681 100644
@@ -71,7 +71,7 @@
 
 struct listeners {
 	struct rcu_head		rcu;
-	unsigned long		masks[0];
+	unsigned long		masks[];
 };
 
 /* state bits */

2.25.0

From: "Gustavo A. R. Silva" <[email protected]>
Date: Mon, 17 Feb 2020 14:07:19 -0600

Applied.