Failed to build against linux-unstable 6.6.0-8-generic: error: 'struct genl_info' has no member named 'userhdr'

[SRU Justification]

[Impact]

backport-iwlwifi-dkms fails to compile against kernel > 6.6 due to API changes.

[Fix]

Upstream fix commit 8fd541f76cdc ("backport: genetlink: adjust to info->userhdr removal").

[Test Case]
```
$ sudo add-apt-repository ppa:canonical-kernel-team/unstable
$ sudo apt-get install linux-headers-generic-wip

$ sudo add-apt-repository ppa:vicamo/ppa-2042059
$ sudo apt-get install backport-iwlwifi-dkms
```

[Where problems could occur]

We're pulling new upstream version with around 200+ commits for backport-iwlwifi-dkms/noble, but only backport the required fix commit for mantic. Likely to expect issue in Noble only.

[Other Info]

Mantic is nominated because we're still using mantic environment for preparing linux-unstable.

========== original bug report ==========

In file included from /var/lib/dkms/backport-iwlwifi/11289/build/drivers/net/wireless/intel/iwlwifi/iwl-tm-gnl.c:8:
/var/lib/dkms/backport-iwlwifi/11289/build/backport-include/net/genetlink.h: In function '__bp_genl_info_userhdr_set':
/var/lib/dkms/backport-iwlwifi/11289/build/backport-include/net/genetlink.h:9:13: error: 'struct genl_info' has no member named 'userhdr'
    9 | info->userhdr = userhdr;
      | ^~
/var/lib/dkms/backport-iwlwifi/11289/build/backport-include/net/genetlink.h: In function '__bp_genl_info_userhdr':
/var/lib/dkms/backport-iwlwifi/11289/build/backport-include/net/genetlink.h:14:20: error: 'struct genl_info' has no member named 'userhdr'
   14 | return info->userhdr;
      | ^~
/var/lib/dkms/backport-iwlwifi/11289/build/backport-include/net/genetlink.h: At top level:
/var/lib/dkms/backport-iwlwifi/11289/build/backport-include/net/genetlink.h:48:21: error: conflicting types for 'genl_info_userhdr'; have 'void *(struct genl_info *)'
   48 | static inline void *genl_info_userhdr(struct genl_info *info)
      | ^~~~~~~~~~~~~~~~~
In file included from /var/lib/dkms/backport-iwlwifi/11289/build/backport-include/net/genetlink.h:3:
./include/net/genetlink.h:126:21: note: previous definition of 'genl_info_userhdr' with type 'void *(const struct genl_info *)'
  126 | static inline void *genl_info_userhdr(const struct genl_info *info)
      | ^~~~~~~~~~~~~~~~~
make[8]: *** [scripts/Makefile.build:251: /var/lib/dkms/backport-iwlwifi/11289/build/drivers/net/wireless/intel/iwlwifi/iwl-tm-gnl.o] Error 1
make[8]: *** Waiting for unfinished jobs....