io_uring: add 'cq_flags' field for the CQ ring

This patch adds the new 'cq_flags' field that should be written by
the application and read by the kernel.

This new field is available to the userspace application through
'cq_off.flags'.
We are using 4-bytes previously reserved and set to zero. This means
that if the application finds this field to zero, then the new
functionality is not supported.

In the next patch we will introduce the first flag available.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>