io_uring/eventfd: move ctx->evfd_last_cq_tail into io_ev_fd

io_uring/eventfd: move ctx->evfd_last_cq_tail into io_ev_fd

Everything else about the io_uring eventfd support is nicely kept
private to that code, except the cached_cq_tail tracking. With
everything else in place, move io_eventfd_flush_signal() to using
the ev_fd grab+release helpers, which then enables the direct use of
io_ev_fd for this tracking too.

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