How do you build this module in-tree?

[tabletseeker]

• Clone kernel: git clone https://git.kernel.org/pub/scm/linux/kernel.....
• Clone dco: git clone https://github.com/OpenVPN/ovpn-dco -b master
• Copy ovpn-dco-master/drivers/net/ovpn-dco to kernel source code /linux/drivers/net/ovpn-dco
• Copy contents of ovpn-dco-master/drivers/net/Kconfig to /linux/drivers/net/Kconfig
• Edit /linux/drivers/net/Makefile to include obj-$(CONFIG_OVPN_DCO_V2) += ovpn-dco-v2.o
• Edit /linux/drivers/net/ovpn-dco/Makefile to include obj-$(CONFIG_OVPN_DCO_V2) := ovpn-dco-v2.o
• Add ovpn-dco-master/include/uapi/linux/ovpn_dco.h to /linux/include/uapi/linux/ovpn_dco.h

This is how it normally works, but I get compilation errors.

drivers/net/ovpn-dco/netlink.c:42:42: error: implicit declaration of function ‘NLA_POLICY_MAX_LEN’; did you mean ‘NLA_POLICY_MIN_LEN’? [-Werror=implicit-function-declaration]
   42 |         [OVPN_KEY_DIR_ATTR_CIPHER_KEY] = NLA_POLICY_MAX_LEN(U8_MAX),
      |                                          ^~~~~~~~~~~~~~~~~~
      |                                          NLA_POLICY_MIN_LEN
drivers/net/ovpn-dco/netlink.c:42:42: error: initializer element is not constant
drivers/net/ovpn-dco/netlink.c:42:42: note: (near initialization for ‘ovpn_netlink_policy_key_dir[1].type’)
drivers/net/ovpn-dco/netlink.c:41:89: warning: missing braces around initializer [-Wmissing-braces]
   41 | static const struct nla_policy ovpn_netlink_policy_key_dir[OVPN_KEY_DIR_ATTR_MAX + 1] = {
      |                                                                                         ^
drivers/net/ovpn-dco/netlink.c:76:41: error: initializer element is not constant
   76 |         [OVPN_NEW_PEER_ATTR_LOCAL_IP] = NLA_POLICY_MAX_LEN(sizeof(struct in6_addr)),
      |                                         ^~~~~~~~~~~~~~~~~~
drivers/net/ovpn-dco/netlink.c:76:41: note: (near initialization for ‘ovpn_netlink_policy_new_peer[6].type’)
drivers/net/ovpn-dco/netlink.c:70:91: warning: missing braces around initializer [-Wmissing-braces]
   70 | static const struct nla_policy ovpn_netlink_policy_new_peer[OVPN_NEW_PEER_ATTR_MAX + 1] = {
      |                                                                                           ^
  CC [M]  drivers/net/ovpn-dco/udp.o
cc1: some warnings being treated as errors
make[8]: *** [scripts/Makefile.build:244: drivers/net/ovpn-dco/netlink.o] Error 1
make[8]: *** Waiting for unfinished jobs....
drivers/net/ovpn-dco/tcp.c:180:5: warning: "LINUX_VERSION_CODE" is not defined, evaluates to 0 [-Wundef]
  180 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 15, 0) && RHEL_RELEASE_CODE < RHEL_RELEASE_VERSION(9, 0) && SUSE_PRODUCT_CODE < SUSE_PRODUCT(1, 15, 5, 0)
      |     ^~~~~~~~~~~~~~~~~~
drivers/net/ovpn-dco/tcp.c:180:26: warning: "KERNEL_VERSION" is not defined, evaluates to 0 [-Wundef]
  180 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 15, 0) && RHEL_RELEASE_CODE < RHEL_RELEASE_VERSION(9, 0) && SUSE_PRODUCT_CODE < SUSE_PRODUCT(1, 15, 5, 0)
      |                          ^~~~~~~~~~~~~~
drivers/net/ovpn-dco/tcp.c:180:40: error: missing binary operator before token "("
  180 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 15, 0) && RHEL_RELEASE_CODE < RHEL_RELEASE_VERSION(9, 0) && SUSE_PRODUCT_CODE < SUSE_PRODUCT(1, 15, 5, 0)
      |                                        ^
drivers/net/ovpn-dco/tcp.c:201:5: warning: "LINUX_VERSION_CODE" is not defined, evaluates to 0 [-Wundef]
  201 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 19, 0)
      |     ^~~~~~~~~~~~~~~~~~
drivers/net/ovpn-dco/tcp.c:201:26: warning: "KERNEL_VERSION" is not defined, evaluates to 0 [-Wundef]
  201 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 19, 0)
      |                          ^~~~~~~~~~~~~~
drivers/net/ovpn-dco/tcp.c:201:40: error: missing binary operator before token "("
  201 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 19, 0)
      |                                        ^
drivers/net/ovpn-dco/tcp.c: In function ‘ovpn_tcp_recvmsg’:
drivers/net/ovpn-dco/tcp.c:213:5: warning: "LINUX_VERSION_CODE" is not defined, evaluates to 0 [-Wundef]
  213 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 19, 0)
      |     ^~~~~~~~~~~~~~~~~~
drivers/net/ovpn-dco/tcp.c:213:26: warning: "KERNEL_VERSION" is not defined, evaluates to 0 [-Wundef]
  213 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 19, 0)
      |                          ^~~~~~~~~~~~~~
drivers/net/ovpn-dco/tcp.c:213:40: error: missing binary operator before token "("
  213 | #if LINUX_VERSION_CODE < KERNEL_VERSION(5, 19, 0)

What's missing here?