netdevice: 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 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>

include/linux/netdevice.h

@@ -664,7 +664,7 @@ static inline void netdev_queue_numa_node_write(struct netdev_queue *q, int node
 struct rps_map {
 	unsigned int len;
 	struct rcu_head rcu;
-	u16 cpus[0];
+	u16 cpus[];
 };
 #define RPS_MAP_SIZE(_num) (sizeof(struct rps_map) + ((_num) * sizeof(u16)))
 
@@ -686,7 +686,7 @@ struct rps_dev_flow {
 struct rps_dev_flow_table {
 	unsigned int mask;
 	struct rcu_head rcu;
-	struct rps_dev_flow flows[0];
+	struct rps_dev_flow flows[];
 };
 #define RPS_DEV_FLOW_TABLE_SIZE(_num) (sizeof(struct rps_dev_flow_table) + \
     ((_num) * sizeof(struct rps_dev_flow)))
@@ -704,7 +704,7 @@ struct rps_dev_flow_table {
 struct rps_sock_flow_table {
 	u32	mask;
 
-	u32	ents[0] ____cacheline_aligned_in_smp;
+	u32	ents[] ____cacheline_aligned_in_smp;
 };
 #define	RPS_SOCK_FLOW_TABLE_SIZE(_num) (offsetof(struct rps_sock_flow_table, ents[_num]))
 
@@ -767,7 +767,7 @@ struct xps_map {
 	unsigned int len;
 	unsigned int alloc_len;
 	struct rcu_head rcu;
-	u16 queues[0];
+	u16 queues[];
 };
 #define XPS_MAP_SIZE(_num) (sizeof(struct xps_map) + ((_num) * sizeof(u16)))
 #define XPS_MIN_MAP_ALLOC ((L1_CACHE_ALIGN(offsetof(struct xps_map, queues[1])) \
@@ -778,7 +778,7 @@ struct xps_map {
  */
 struct xps_dev_maps {
 	struct rcu_head rcu;
-	struct xps_map __rcu *attr_map[0]; /* Either CPUs map or RXQs map */
+	struct xps_map __rcu *attr_map[]; /* Either CPUs map or RXQs map */
 };
 
 #define XPS_CPU_DEV_MAPS_SIZE(_tcs) (sizeof(struct xps_dev_maps) +	\