LinuxKPI: Add cancel_work() function.
Cancel a work not waiting for it to finish.
Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42811
(cherry picked from commit 1b2f43a7427ebf51561867f6c497833268014512)