LinuxKPI: Convert Linux integer types to ISO C99 in linux/kstrtox.h

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: bz, emaste
Differential Revision: https://reviews.freebsd.org/D45465

(cherry picked from commit db42bd1f71b1a29b392aacbd5b7aaca4c8d7e85d)