utimens.c: correct kernel bug work-around

Ondřej Vašík found that the invalid return value of 280 indicates
failure, not success, and the kernel bug we're trying to work
around affects not just the utimensat call, but also the fallback
futimens call.
* lib/utimens.c (gl_futimens) [HAVE_UTIMENSAT]: Simulate failure,
not success.
[HAVE_FUTIMENS]: Use the same work-around, here.