Documentation: Replace more lkml.org links with lore

Documentation: Replace more lkml.org links with lore

As started by commit 05a5f51ca566 ("Documentation: Replace lkml.org
links with lore"), replace a few more scattered lkml.org links with
lore to better use a single source that's more likely to stay available
long-term.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210210234005.2236201-1-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>