Check field regardless of whether TUNNEL_CSUM is configured. Link: https://lore.kernel.org/netdev/YS+h%2FtqCJJiQei+W@shredder/ Link: https://lore.kernel.org/all/20210902193447.94039-2-willemdebruijn.kernel@gmail.com/T/#u Fixes: 8a0ed25 ("ip_gre: validate csum_start only on pull")...