net/mlx5e: Replace zero-length arrays with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations in anonymous union with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/222
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.htmlSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

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

@@ -97,8 +97,8 @@ struct mlx5_flow_attr {
 	} lag;
 	/* keep this union last */
 	union {
-		struct mlx5_esw_flow_attr esw_attr[0];
-		struct mlx5_nic_flow_attr nic_attr[0];
+		DECLARE_FLEX_ARRAY(struct mlx5_esw_flow_attr, esw_attr);
+		DECLARE_FLEX_ARRAY(struct mlx5_nic_flow_attr, nic_attr);
 	};
 };