io_uring/io-wq: only free worker if it was allocated for creation

We have two types of task_work based creation, one is using an existing
worker to setup a new one (eg when going to sleep and we have no free
workers), and the other is allocating a new worker. Only the latter
should be freed when we cancel task_work creation for a new worker.

Fixes: af82425c ("io_uring/io-wq: free worker if task_work creation is canceled")
Reported-by:  <syzbot+d56ec896af3637bdb7e4@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>