io_uring: Adjust mapping wrt architecture aliasing requirements

Some architectures have memory cache aliasing requirements (e.g. parisc)
if memory is shared between userspace and kernel. This patch fixes the
kernel to return an aliased address when asked by userspace via mmap().

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>