lib: objagg: Replace zero-length arrays 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
inadvertenly introduced[3] to the codebase from now on.

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 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/objagg.c

@@ -28,7 +28,7 @@ struct objagg_hints_node {
 	struct objagg_hints_node *parent;
 	unsigned int root_id;
 	struct objagg_obj_stats_info stats_info;
-	unsigned long obj[0];
+	unsigned long obj[];
 };
 
 static struct objagg_hints_node *
@@ -66,7 +66,7 @@ struct objagg_obj {
 				* including nested objects
 				*/
 	struct objagg_obj_stats stats;
-	unsigned long obj[0];
+	unsigned long obj[];
 };
 
 static unsigned int objagg_obj_ref_inc(struct objagg_obj *objagg_obj)