io_uring/uring_cmd: implement ->sqe_copy() to avoid unnecessary copies

io_uring/uring_cmd: implement ->sqe_copy() to avoid unnecessary copies

uring_cmd currently copies the full SQE at prep time, just in case it
needs it to be stable. However, for inline completions or requests that
get queued up on the device side, there's no need to ever copy the SQE.
This is particularly important, as various use cases of uring_cmd will
be using 128b sized SQEs.

Opt in to using ->sqe_copy() to let the core of io_uring decide when to
copy SQEs. This callback will only be called if it is safe to do so.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>