linuxkpi: Add <linux/mman.h> which includes <linux/smp.h>

This is used by i915_gem.c in the i915 DRM driver to get access to
wbinvd_on_all_cpus().

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D36970

(cherry picked from commit 24c7853f39d0f68d92f97880df2428f9e78e165d)