LinuxKPI: Add IOMEM_ERR_PTR() to linux/io.h

The function creates an error pointer.

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42805

(cherry picked from commit af787b8e8b803dbb2c6bd06629974ba39bd0fb70)