net/mlx5e: 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 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>

drivers/net/ethernet/mellanox/mlx5/core/en.h

@@ -204,7 +204,7 @@ struct mlx5e_tx_wqe {
 
 struct mlx5e_rx_wqe_ll {
 	struct mlx5_wqe_srq_next_seg  next;
-	struct mlx5_wqe_data_seg      data[0];
+	struct mlx5_wqe_data_seg      data[];
 };
 
 struct mlx5e_rx_wqe_cyc {

drivers/net/ethernet/mellanox/mlx5/core/fpga/ipsec.c

@@ -57,7 +57,7 @@ struct mlx5_fpga_ipsec_cmd_context {
 	struct completion complete;
 	struct mlx5_fpga_device *dev;
 	struct list_head list; /* Item in pending_cmds */
-	u8 command[0];
+	u8 command[];
 };
 
 struct mlx5_fpga_esp_xfrm;

drivers/net/ethernet/mellanox/mlx5/core/fs_counters.c

@@ -470,7 +470,7 @@ struct mlx5_fc_bulk {
 	u32 base_id;
 	int bulk_len;
 	unsigned long *bitmask;
-	struct mlx5_fc fcs[0];
+	struct mlx5_fc fcs[];
 };
 
 static void mlx5_fc_init(struct mlx5_fc *counter, struct mlx5_fc_bulk *bulk,

drivers/net/ethernet/mellanox/mlx5/core/ipoib/ipoib.h

@@ -56,7 +56,7 @@ struct mlx5i_priv {
 	u32    qkey;
 	u16    pkey_index;
 	struct mlx5i_pkey_qpn_ht *qpn_htbl;
-	char  *mlx5e_priv[0];
+	char  *mlx5e_priv[];
 };
 
 int mlx5i_create_tis(struct mlx5_core_dev *mdev, u32 underlay_qpn, u32 *tisn);
@@ -107,7 +107,7 @@ struct mlx5i_tx_wqe {
 	struct mlx5_wqe_datagram_seg datagram;
 	struct mlx5_wqe_eth_pad      pad;
 	struct mlx5_wqe_eth_seg      eth;
-	struct mlx5_wqe_data_seg     data[0];
+	struct mlx5_wqe_data_seg     data[];
 };
 
 static inline void mlx5i_sq_fetch_wqe(struct mlx5e_txqsq *sq,