kasan: do not sanitize kexec purgatory

Fixes this:

  kexec: Undefined symbol: __asan_load8_noabort
  kexec-bzImage64: Loading purgatory failed

Link: http://lkml.kernel.org/r/1489672155.4458.7.camel@gmx.de


Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>