io_uring: account locked memory before potential error case

The tear down path will always unaccount the memory, so ensure that we
have accounted it before hitting any of them.

Reported-by: Tomáš Chaloupka <chalucha@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>