tls: strp: rename and multithread the workqueue

Paolo points out that there seems to be no strong reason strparser
users a single threaded workqueue. Perhaps there were some performance
or pinning considerations? Since we don't know (and it's the slow path)
let's default to the most natural, multi-threaded choice.

Also rename the workqueue to "tls-".

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>