mlxsw: core_acl_flex_actions: Split memcpy() of struct flow_action_cookie flexible array

To work around a misbehavior of the compiler's ability to see into
composite flexible array structs (as detailed in the coming memcpy()
hardening series[1]), split the memcpy() of the header and the payload
so no false positive run-time overflow warning will be generated.

[1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org

Cc: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20220927004033.1942992-1-keescook@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -737,8 +737,9 @@ mlxsw_afa_cookie_create(struct mlxsw_afa *mlxsw_afa,
 	if (!cookie)
 		return ERR_PTR(-ENOMEM);
 	refcount_set(&cookie->ref_count, 1);
-	memcpy(&cookie->fa_cookie, fa_cookie,
-	       sizeof(*fa_cookie) + fa_cookie->cookie_len);
+	cookie->fa_cookie = *fa_cookie;
+	memcpy(cookie->fa_cookie.cookie, fa_cookie->cookie,
+	       fa_cookie->cookie_len);
 
 	err = rhashtable_insert_fast(&mlxsw_afa->cookie_ht, &cookie->ht_node,
 				     mlxsw_afa_cookie_ht_params);