io_uring: don't do opcode prep twice

Calling into opcode prep handlers may be dangerous, as they re-read
SQE but might not re-initialise requests completely. If io_req_defer()
passed fast checks and is done with preparations, punt it async.

As all other cases are covered with nulling @sqe, this guarantees that
io_[opcode]_prep() are visited only once per request.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>