Allow KASAN and HOTPLUG_MEMORY to co-exist when doing build testing

No, KASAN may not be able to co-exist with HOTPLUG_MEMORY at runtime,
but for build testing there is no reason not to allow them together.

This hopefully means better build coverage and fewer embarrasing silly
problems like the one fixed by commit 9db4f36e ("mm: remove unused
variable in memory hotplug") in the future.

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>