linuxkpi: Add sysfs_add_file_to_group() and sysfs_remove_file_from_group()

They are used by the amdgpu DRM driver for quite some time, but new code
using them added to Linux 6.9 made me discover that these functions were
missing and existing code was commentted out.

Reviewed by: manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50987