linuxkpi: Add kmemleak.h

Provide macros needed for drm-kmod.

Obtain from: OpenBSD (via drm-kmod)
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35564

(cherry picked from commit 8730ba418e050f622c32766562366729b4d775ad)