epoll: avoid orphaned interest on EventRegister failure

[ibondarenko1]

Problem

EpollInstance.AddInterest inserts the new epollInterest into ep.interest before registering with the file, and adds it to file.epolls only after registration succeeds:

ep.interest[key] = epi
wmask := waiter.EventMaskFromLinux(mask)
epi.waiter.Init(epi, wmask)
if err := file.EventRegister(&epi.waiter); err != nil {
	return err
}
...
file.epolls[epi] = struct{}{}

If file.EventRegister fails, AddInterest returns with epi in ep.interest but never in file.epolls.

The interest-removal paths rely on ep.interest and file.epolls staying in sync: EpollInstance.Release walks ep.interest, and the FileDescription.DecRef path walks file.epolls. An entry present in ep.interest but not file.epolls is orphaned, and it keeps the target *FileDescription reachable past its own release.

EventRegister fails deterministically for an unprivileged caller. For example, epoll_ctl(EPOLL_CTL_ADD) on an opened-but-unmounted /dev/fuse FD reaches fuse.DeviceFD.EventRegister, which returns EPERM when the device is not connected.

Change

Move ep.interest[key] = epi to after EventRegister succeeds. epi is then added to ep.interest and file.epolls only on the success path, with no fallible call between the two inserts, so a failed registration leaves nothing behind. A short comment records the ordering requirement.

Scope

Hardening. A bounded per-epoll_ctl leak plus a stale *FileDescription reference, reachable by an unprivileged process.