MAINTAINERS: change hardening mailing list

As more email from git history gets aimed at the OpenWall
kernel-hardening@ list, there has been a desire to separate "new topics"
from "on-going" work.

To handle this, the superset of hardening email topics are now to be
directed to linux-hardening@vger.kernel.org.

Update the MAINTAINERS file and the .mailmap to accomplish this, so that
linux-hardening@ can be treated like any other regular upstream kernel
development list.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: "Tobin C. Harding" <me@tobin.cc>
Cc: Tycho Andersen <tycho@tycho.pizza>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/linux-hardening/202010051443.279CC265D@keescook/
Link: https://lkml.kernel.org/r/20201006000012.2768958-1-keescook@chromium.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>