io_uring: Use ERR_CAST() instead of ERR_PTR(PTR_ERR())

Use ERR_CAST() instead of ERR_PTR(PTR_ERR()).
This makes it more readable and also fix this warning detected by
err_cast.cocci:
./fs/io_uring.c: WARNING: 3208: 11-18: ERR_CAST can be used with buf
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Link: https://lore.kernel.org/r/20211020084948.1038420-1-deng.changcheng@zte.com.cnSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3172,7 +3172,7 @@ static struct iovec *__io_import_iovec(int rw, struct io_kiocb *req,
 		if (req->flags & REQ_F_BUFFER_SELECT) {
 			buf = io_rw_buffer_select(req, &sqe_len, issue_flags);
 			if (IS_ERR(buf))
-				return ERR_PTR(PTR_ERR(buf));
+				return ERR_CAST(buf);
 			req->rw.len = sqe_len;
 		}