docs: kernel-hacking: be more civil

Remove the f-bomb from locking.rst.  Let's have a moment of silence,
though, as we mark the passing of the last of Rusty's once plentiful
profanities in this venerable document.

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Link: https://lore.kernel.org/r/20210205115951.1276526-1-unixbhaskar@gmail.com
[jc: rewrote changelog]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>