io_uring/rw: clean up __io_read() obsolete comment and early returns

io_uring/rw: clean up __io_read() obsolete comment and early returns

After commit a9165b83c193 ("io_uring/rw: always setup io_async_rw for
read/write requests") which moved the iovec allocation into the prep
path and stores it in req->async_data where it now gets freed as part of
the request lifecycle, this comment is now outdated.

Remove it and clean up the goto as well.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20260401173511.4052303-1-joannelkoong@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>