LinuxKPI: Move kstrto* functions to linux/kstrtox.h

from linux/kernel.h to match Linux and
allow direct linux/kstrtox.h inclusion.

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D45454

(cherry picked from commit a97f3a9ec96a2de542326a158d1ab8e65ad4fe5f)