eventfd: Export eventfd_wake_count to modules

Export eventfd_wake_count so that some modules can use
the eventfd_signal_count() to check whether the
eventfd_signal() call should be deferred to a safe context.

NB(mst): this patch is not needed in Linus tree since there
eventfd_signal_count() has been superseded by an already exported
eventfd_signal_allowed().

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210831103634.33-3-xieyongji@bytedance.com
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>