checkpatch: remove warning for 'old' stable@kernel.org address

checkpatch: remove warning for 'old' stable@ address

It may not be the actual real stable mailing list address, but the
stable scripts to actually pick up on the traditional way to mark stable
patches.

There are also reasons to explicitly avoid using the actual mailing list
address, since security patches with embargo dates generally do want the
stable marking, but don't want tools etc to mistakenly send the patch
out to the mailing list early.

So don't warn for things that are still actively used and explicitly
supported.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>