x86/Documentation: Update the contact email for L3 cache index disable functionality

The mailing list discuss@x86-64.org is now defunct.
Use the lkml in its place.

Signed-off-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Greg KH <greg@kroah.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-api@vger.kernel.org
Cc: sboyd@codeaurora.org
Cc: sudeep.holla@arm.com
Link: http://lkml.kernel.org/r/1431125098-9470-1-git-send-email-Aravind.Gopalakrishnan@amd.com
Link: http://lkml.kernel.org/r/1431936437-25286-2-git-send-email-bp@alien8.de
[ Changed the contact email to lkml. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>