LinuxKPI: Add min_not_zero() to linux/minmax.h

It returns the minimum that is not zero, except both equals to zero.

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: bz, emaste, ssaxena
Differential Revision: https://reviews.freebsd.org/D45450
Differential Revision: https://reviews.freebsd.org/D45451