io_uring: don't dynamically allocate poll data

io_uring: don't dynamically allocate poll data

This essentially reverts commit 9126401b94e5. For high poll ops
workloads, like TAO, the dynamic allocation of the wait_queue
entry for IORING_OP_POLL_ADD adds considerable extra overhead.
Go back to embedding the wait_queue_entry, but keep the usage of
wait->private for the pointer stashing.

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