neighbour: 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")

---
 include/net/neighbour.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 8ec77bfdc1a4..e1476775769c 100644
@@ -174,7 +174,7 @@ struct pneigh_entry {
 	struct net_device	*dev;
 	u8			flags;
 	u8			protocol;
-	u8			key[0];
+	u8			key[];
 };
 
 /*

2.25.0

From: "Gustavo A. R. Silva" <[email protected]>
Date: Fri, 28 Feb 2020 19:05:02 -0600

Applied.