io_uring/nop: add support for IORING_SETUP_CQE_MIXED

io_uring/nop: add support for IORING_SETUP_CQE_MIXED

This adds support for setting IORING_NOP_CQE32 as a flag for a NOP
command, in which case a 32b CQE will be posted rather than a regular
one. This is the default if the ring has been setup with
IORING_SETUP_CQE32. If the ring has been setup with
IORING_SETUP_CQE_MIXED, then 16b CQEs will be posted without this flag
set, and 32b CQEs if this flag is set. For the latter case, sqe->off is
what will be posted as cqe->big_cqe[0] and sqe->addr is what will be
posted as cqe->big_cqe[1].

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