LinuxKPI: stub anon_inode_getfile

Although drm-kmod contains better implementation which is able to
allocate real entries on pseudofs, this feature has never been used.

Starting from drm-kmod v5.6 old implementation began to leak entries
on each drm device close(). Now just drop pseudofs support instead of
fixing it in drm-kmod and provide stub in base.

Reviewed by: hselasky, manu
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D32069