LinuxKPI: Add get_random_u32_below function

get_random_u32_below returns a random integer in the interval [0, ceil),
with uniform distribution.

Sponsored by: Serenity CyberSecurity, LLC
Reviewed by: emaste
MFC after: 1 week

(cherry picked from commit 9289c1f6f1514e714b17026c50ec09282f079cba)