ceph: use cap_wait_list only if debugfs is enabled

Only debugfs uses this list.  By omitting it, we save some memory and
reduce lock contention on `caps_list_lock`.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>