io_uring/eventfd: abstract out ev_fd grab + release helpers

In preparation for needing the ev_fd grabbing (and releasing) from
another path, abstract out two helpers for that.

Link: https://lore.kernel.org/r/20240921080307.185186-6-axboe@kernel.dk


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