mm/damon/vaddr-test: fix memory leak in damon_do_test_apply_three_regions()

When CONFIG_DAMON_VADDR_KUNIT_TEST=y and making CONFIG_DEBUG_KMEMLEAK=y
and CONFIG_DEBUG_KMEMLEAK_AUTO_SCAN=y, the below memory leak is detected.

Since commit 9f86d624 ("mm/damon/vaddr-test: remove unnecessary
variables"), the damon_destroy_ctx() is removed, but still call
damon_new_target() and damon_new_region(), the damon_region which is
allocated by kmem_cache_alloc() in damon_new_region() and the damon_target
which is allocated by kmalloc in damon_new_target() are not freed.  And
the damon_region which is allocated in damon_new_region() in
damon_set_regions() is also not freed.

So use damon_destroy_target to free all the damon_regions and damon_target.

    unreferenced object 0xffff888107c9a940 (size 64):
      comm "kunit_try_catch", pid 1069, jiffies 4294670592 (age 732.761s)
      hex dump (first 32 bytes):
        00 00 00 00 00 00 00 00 06 00 00 00 6b 6b 6b 6b  ............kkkk
        60 c7 9c 07 81 88 ff ff f8 c...

mm/damon/vaddr-test.h

@@ -148,6 +148,8 @@ static void damon_do_test_apply_three_regions(struct kunit *test,
 		KUNIT_EXPECT_EQ(test, r->ar.start, expected[i * 2]);
 		KUNIT_EXPECT_EQ(test, r->ar.end, expected[i * 2 + 1]);
 	}
+
+	damon_destroy_target(t);
 }
 
 /*