Remove epoll entry from map when dropping it.

This pattern (delete from map when dropping) is also used in epoll.RemoveEntry,
and seems like generally a good idea.

PiperOrigin-RevId: 291268208

pkg/sentry/kernel/epoll/epoll.go

@@ -174,6 +174,7 @@ func (e *EventPoll) Release() {
 		entry.id.File.EventUnregister(&entry.waiter)
 		entry.file.Drop()
 	}
+	e.files = nil
 }
 
 // Read implements fs.FileOperations.Read.