LinuxKPI: Move linux/minmax.h content out of linux/kernel.h
To match Linux and allow direct linux/minmax.h inclusion.
Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D45449
(cherry picked from commit f79474c4e3eeb48c6acb0a9e3163195a50cef577)