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.cn


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