kunit: Fix the wrong kfree of copy for kunit_filter_suites()

If the outer layer for loop is iterated more than once and it fails not
in the first iteration, the copy pointer has been moved. So it should free
the original copy's backup copy_start.

Fixes: abbf7381

 ("kunit: fix possible memory leak in kunit_filter_suites()")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>