io_uring/rw: don't attempt to allocate async data if opcode doesn't need it

The new read multishot method doesn't need to allocate async data ever,
as it doesn't do vectored IO and it must only be used with provided
buffers. While it doesn't have ->prep_async() set, it also sets
->async_size to 0, which is different from any other read/write type we
otherwise support.

If it's used on a file type that isn't pollable, we do try and allocate
this async data, and then try and use that data. But since we passed in
a size of 0 for the data, we get a NULL back on data allocation. We then
proceed to dereference that to copy state, and that obviously won't end
well.

Add a check in io_setup_async_rw() for this condition, and avoid copying
state. Also add a check for whether or not buffer selection is specified
in prep while at it.

Fixes: fc68fcda ("io_uring/rw: add support for IORING_OP_READ_MULTISHOT")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218101


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