io_uring: do poll's hash_node init in common code

Move INIT_HLIST_NODE(&req->hash_node) into __io_arm_poll_handler(), so
that it doesn't duplicated and common poll code would be responsible for
it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>