io_uring: don't attempt to copy iovec for READ/WRITE

For the non-vectored variant of READV/WRITEV, we don't need to setup an
async io context, and we flag that appropriately in the io_op_defs
array. However, in fixing this for the 5.5 kernel in commit 74566df3a7
we didn't have these opcodes, so the check there was added just for the
READ_FIXED and WRITE_FIXED opcodes. Replace that check with just a
single check for needing async context, that covers all four of these
read/write variants that don't use an iovec.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2112,8 +2112,7 @@ static int io_setup_async_rw(struct io_kiocb *req, ssize_t io_size,
 			     struct iovec *iovec, struct iovec *fast_iov,
 			     struct iov_iter *iter)
 {
-	if (req->opcode == IORING_OP_READ_FIXED ||
-	    req->opcode == IORING_OP_WRITE_FIXED)
+	if (!io_op_defs[req->opcode].async_ctx)
 		return 0;
 	if (!req->io && io_alloc_async_ctx(req))
 		return -ENOMEM;