locking/csd_lock: Prepare more CSD lock debugging

In order to be able to easily add more CSD lock debugging data to
struct call_function_data->csd move the call_single_data_t element
into a sub-structure.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210301101336.7797-3-jgross@suse.com