linuxkpi: Alias sd field of struct kobject

On Linux, a struct kernfs_node * pointer, representing a directory in
sysfs, is kept in the sd struct field.

We don't have that on FreeBSD because we use sysctls instead. Let's
alias the sysctl OID pointer to sd using an union.

This pointer is checked by the DRM drivers using:

if (var->kobj.sd) {
    ...
}

The amdgpu DRM driver started to use these checks in Linux 6.13.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57586