io_uring: allow POLL_ADD with double poll_wait() users

io_uring: allow POLL_ADD with double poll_wait() users

Some file descriptors use separate waitqueues for their f_ops->poll()
handler, most commonly one for read and one for write. The io_uring
poll implementation doesn't work with that, as the 2nd poll_wait()
call will cause the io_uring poll request to -EINVAL.

This is particularly a problem now that pipes were switched to using
multiple wait queues (commit 0ddad21d3e99), but it also affects tty
devices and /dev/random as well. This is a big problem for event loops
where some file descriptors work, and others don't.

With this fix, io_uring handles multiple waitqueues.

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