io_uring: add helper for parsing poll events

Isolate poll mask SQE parsing and preparations into a new function,
which will be reused shortly.

Fixes: b69de288 ("io_uring: allow events and user_data update of running poll requests")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>