fs/aio: Check IOCB_AIO_RW before the struct aio_kiocb conversion

In the Linux kernel, the following vulnerability has been resolved: fs/aio: Check IOCB_AIO_RW before the struct aio_kiocb conversion The first kiocb_set_cancel_fn() argument may point at a struct kiocb that is not embedded inside struct aio_kiocb. With the current code, depending on the compiler, the req->ki_ctx read happens either before the IOCB_AIO_RW test or after that test. Move the req->ki_ctx read such that it is guaranteed that the IOCB_AIO_RW test happens first.