eventfd: provide a eventfd_signal_mask() helper

This is identical to eventfd_signal(), but it allows the caller to pass
in a mask to be used for the poll wakeup key. The use case is avoiding
repeated multishot triggers if we have a dependency between eventfd and
io_uring.

If we setup an eventfd context and register that as the io_uring eventfd,
and at the same time queue a multishot poll request for the eventfd
context, then any CQE posted will repeatedly trigger the multishot request
until it terminates when the CQ ring overflows.

In preparation for io_uring detecting this circular dependency, add the
mentioned helper so that io_uring can pass in EPOLL_URING as part of the
poll wakeup key.

Cc: stable@vger.kernel.org # 6.0
[axboe: fold in !CONFIG_EVENTFD fix from Zhang Qilong]
Signed-off-by: Jens Axboe <axboe@kernel.dk>