This lets it happen before per-request cancellation, which otherwise
causes all future ops to start cancelled after one finishes.

An interrupt for the kernel will cause the original op's context to be
cancelled.

For GoogleCloudPlatform/gcsfuse#41.

The user must already have this checking, so we don't need it. Getting rid of
it will allow us to call beginOp before calling Convert, necessary for putting
aditional state keeping in that method.