mm/damon/core-test: fix damon_test_ops_registration() for DAMON_VADDR unset case

mm/damon/core-test: fix damon_test_ops_registration() for DAMON_VADDR unset case

DAMON core kunit test can be executed without CONFIG_DAMON_VADDR.  In the
case, vaddr DAMON ops is not registered.  Meanwhile, ops registration
kunit test assumes the vaddr ops is registered.  Check and handle the case
by registrering fake vaddr ops inside the test code.

Link: https://lkml.kernel.org/r/20240827030336.7930-6-sj@kernel.org
Fixes: 4f540f5ab4f2 ("mm/damon/core-test: add a kunit test case for ops registration")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>