io_uring/uring_cmd: copy SQE only when needed

io_uring/uring_cmd: copy SQE only when needed

If the request is flagged with REQ_F_FORCE_ASYNC or REQ_F_ASYNC_PREP,
then there's a chance that it will get issued out-of-line. For that case,
the SQE must be copied.

Add an SQE copy helper, and use it on the prep side if the request is
flagged as such, and from the main issue path if we get -EAGAIN when
attempting to issue the request.

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