io-wq: remove nr_process accounting

We're now just using fork like we would from userspace, so there's no
need to try and impose extra restrictions or accounting on the user
side of things. That's already being done for us. That also means we
don't have to pass in the user_struct anymore, that's correctly inherited
through ->creds on fork.

Signed-off-by: Jens Axboe <axboe@kernel.dk>