LinuxKPI: Move kfree_async() functionality in to kfree()

Obsolete it usage but keep for a while for drm-kmod 5.4 compatibility

MFC after: 1 week
Reviewed by: hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33298

(cherry picked from commit 0b1244bd79c2c4da25278a2e25a2471d9edd9408)