io_uring/eventfd: move to more idiomatic RCU free usage

In some ways, it just "happens to work" currently with using the ops
field for both the free and signaling bit. But it depends on ordering
of operations in terms of freeing and signaling. Clean it up and use the
usual refs == 0 under RCU read side lock to determine if the ev_fd is
still valid, and use the reference to gate the freeing as well.

Fixes: 21a091b9

 ("io_uring: signal registered eventfd to process deferred task work")
Signed-off-by: Jens Axboe <axboe@kernel.dk>