LinuxKPI: Add linux/nodemask.h

and dummy num_possible_nodes() implementation.
We do not fully support NUMA in LinuxKPI yet.

Sponsored by: Serenity CyberSecurity, LLC
MFC after: 1 week
Reviewed by: manu, bz
Differential Revision: https://reviews.freebsd.org/D45613

(cherry picked from commit 889f74af66b82e4401fb69ed8af6b9b4f48ce53b)