Don't eat error from epoll_ctl EPOLL_CTL_ADD

Docker maps stdin to `/dev/null` which doesn't support epoll. Host FD
was ignoring the error and suceeding the epoll_ctl call from the
container, giving false impressing that epoll would be notified.

This required plumbing failure to all waiter.Waitable.EventRegister
callers and implementers.

Closes #6795

PiperOrigin-RevId: 414797621

pkg/sentry/socket/netstack/netstack.go

@@ -3393,4 +3393,10 @@ func (s *socketOpsCommon) Type() (family int, skType linux.SockType, protocol in
 	return s.family, s.skType, s.protocol
 }
 
+// EventRegister implements waiter.Waitable.
+func (s *socketOpsCommon) EventRegister(e *waiter.Entry) error {
+	s.Queue.EventRegister(e)
+	return nil
+}
+
 // LINT.ThenChange(./netstack_vfs2.go)

pkg/sentry/socket/netstack/netstack_vfs2.go

@@ -90,8 +90,9 @@ func (s *SocketVFS2) Readiness(mask waiter.EventMask) waiter.EventMask {
 }
 
 // EventRegister implements waiter.Waitable.EventRegister.
-func (s *SocketVFS2) EventRegister(e *waiter.Entry) {
+func (s *SocketVFS2) EventRegister(e *waiter.Entry) error {
 	s.socketOpsCommon.EventRegister(e)
+	return nil
 }
 
 // EventUnregister implements waiter.Waitable.EventUnregister.