linuxkpi: Add stub kmem_cache_shrink
Needed by drm-kmod.
Reviewed by: bz
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36108
(cherry picked from commit 35b7625ed026d8afaf3e8e27bc9d1067d38dbde6)