another fix for "kasan: improve vmalloc tests"

another fix for "kasan: improve vmalloc tests"

set_memory_rw/ro() are not exported to be used in modules and thus
cannot be used in KUnit-compatible KASAN tests.

Do the checks that rely on these functions only when the tests are
built-in.

Link: https://lkml.kernel.org/r/019ac41602e0c4a7dfe96dc8158a95097c2b2ebd.1645554036.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>