linuxkpi: Add mapping_clear_unevictable stub

Reviewed by: hselasky
Obtained from: drm-kmod
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35939

(cherry picked from commit 4aa82e4358c0cefffbcc30ef63bfb787ffc6030c)