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.

(cherry picked from commit b382b78503b56ad6b787b995b46418db27adaa61)