io_uring: don't pass def into io_req_work_grab_env

io_uring: don't pass def into io_req_work_grab_env

Remove struct io_op_def *def parameter from io_req_work_grab_env(),
it's trivially deducible from req->opcode and fast. The API is
cleaner this way, and also helps the complier to understand
that it's a real constant and could be register-cached.

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