io_uring: mask in error/nval/hangup consistently for poll

Instead of masking these in as part of regular POLL_ADD prep, do it in
io_init_poll_iocb(), and include NVAL as that's generally unmaskable,
and RDHUP alongside the HUP that is already set.

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