dm crypt: export sysfs of kcryptd workqueue

It should be helpful to export sysfs of "kcryptd" workqueue in some
cases, such as setting specific CPU affinity of the workqueue.

Besides, also tweak the name format a little. The slash inside a
directory name will be translate into exclamation mark, such as
/sys/devices/virtual/workqueue/'kcryptd!253:0'.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>