net: core: 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>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/bpf_sk_storage.c

@@ -60,7 +60,7 @@ struct bpf_sk_storage_data {
 	 * the number of cachelines access during the cache hit case.
 	 */
 	struct bpf_sk_storage_map __rcu *smap;
-	u8 data[0] __aligned(8);
+	u8 data[] __aligned(8);
 };
 
 /* Linked to bpf_sk_storage and bpf_sk_storage_map */

net/core/devlink.c

@@ -4232,7 +4232,7 @@ struct devlink_fmsg_item {
 	int attrtype;
 	u8 nla_type;
 	u16 len;
-	int value[0];
+	int value[];
 };
 
 struct devlink_fmsg {

net/core/drop_monitor.c

@@ -68,7 +68,7 @@ struct net_dm_hw_entry {
 
 struct net_dm_hw_entries {
 	u32 num_entries;
-	struct net_dm_hw_entry entries[0];
+	struct net_dm_hw_entry entries[];
 };
 
 struct per_cpu_dm_data {