[PATCH] async-thread: avoid closing eventfd twice

[PATCH] async-thread: avoid closing eventfd twice

When employing eventfd for socketpair, there is only one file
descriptor. Closing that fd twice might result in fd corruption.
Thus, we should avoid closing the eventfd twice, following the
pattern in lib/multi.c.

Fixes #15725
Closes #15727
Reported-by: Christian Heusel
Gbp-Pq: Name async_thread_avoid_closing_eventfd_twice.patch