io_uring: test patch for fd passing

io_uring: test patch for fd passing

This allows a chain link to set ->fd to IOSQE_FD_LAST_OPEN, which will
then be turned into the results from the last open (or accept) request
in the chain. If no open has been done, this isn't valid and the request
will be errored with -EBADF.

With this, we can define chains of open+read+close, where the read and
close part can work on the fd instantiated by the open request.

Totally a work in progress, POC so far.

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