io_uring/eventfd: open code io_eventfd_grab()

io_eventfd_grab() doesn't help wit understanding the path, it'll be
simpler to keep the helper open coded.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5cb53ce3876c2819db9e8055cf41dca4398521db.1745493845.git.asml.silence@gmail.com


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