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

[ Upstream commit 08ca27d027c238ed3f9b9968d349cebde44d99a6 ]

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 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: ed779fe4c9b5 ("neighbour: fix unaligned access to pneigh_entry")
Signed-off-by: Sasha Levin <sashal@kernel.org>

include/net/neighbour.h

@@ -174,7 +174,7 @@ struct pneigh_entry {
 	struct net_device	*dev;
 	u8			flags;
 	u8			protocol;
-	u8			key[0];
+	u8			key[];
 };
 
 /*