LinuxKPI: add kstrtou8() and kstrtou8_from_user() to kernel.h

Analogous to the other sized version of kstrto[u]<type>() and
kstrtobool_from_user() add the "u8" versions needed by a driver.

MFC after: 3 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D32598