io_uring: use better types for cflags

__io_cqring_fill_event() takes cflags as long to squeeze it into u32 in
an CQE, awhile all users pass int or unsigned. Replace it with unsigned
int and store it as u32 in struct io_completion to match CQE.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>