xdp: 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>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/xdp/xsk_queue.h

@@ -19,13 +19,13 @@ struct xdp_ring {
 /* Used for the RX and TX queues for packets */
 struct xdp_rxtx_ring {
 	struct xdp_ring ptrs;
-	struct xdp_desc desc[0] ____cacheline_aligned_in_smp;
+	struct xdp_desc desc[] ____cacheline_aligned_in_smp;
 };
 
 /* Used for the fill and completion queues for buffers */
 struct xdp_umem_ring {
 	struct xdp_ring ptrs;
-	u64 desc[0] ____cacheline_aligned_in_smp;
+	u64 desc[] ____cacheline_aligned_in_smp;
 };
 
 struct xsk_queue {