mlxsw: spectrum: acl: Use struct_size() in kzalloc()

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct foo {
    int stuff;
    struct boo entry[];
};

size = sizeof(struct foo) + count * sizeof(struct boo);
instance = kzalloc(size, GFP_KERNEL)

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL)

Notice that, in this case, variable alloc_size is not necessary, hence
it is removed.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlxsw/core_acl_flex_keys.c

@@ -236,12 +236,10 @@ mlxsw_afk_key_info_create(struct mlxsw_afk *mlxsw_afk,
 			  struct mlxsw_afk_element_usage *elusage)
 {
 	struct mlxsw_afk_key_info *key_info;
-	size_t alloc_size;
 	int err;
 
-	alloc_size = sizeof(*key_info) +
-		     sizeof(key_info->blocks[0]) * mlxsw_afk->max_blocks;
-	key_info = kzalloc(alloc_size, GFP_KERNEL);
+	key_info = kzalloc(struct_size(key_info, blocks, mlxsw_afk->max_blocks),
+			   GFP_KERNEL);
 	if (!key_info)
 		return ERR_PTR(-ENOMEM);
 	err = mlxsw_afk_picker(mlxsw_afk, key_info, elusage);