io_uring: poll: remove unnecessary req->ref set

We now don't need to set req->refcount for poll requests since the
reworked poll code ensures no request release race.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ec6fee45705890bdb968b0c175519242753c0215.1655371007.git.asml.silence@gmail.com


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