[sgen] Fix accidental triggering of degraded mode

[sgen] Fix accidental triggering of degraded mode

We trigger degraded mode when we are unable to properly build the nursery fragments due to excessive nursery pinning. This is done by initializing degraded_mode at the end of a minor. alloc_degraded is used to do allocation to major heap either as part of mature allocation or as part of degraded mode (when we bump the degraded_mode so we can detect when the next minor should be triggered).

When failing to allocate from the nursery after doing a minor (because the other threads might have already consumed the nursery) we need to do a mature allocation (as a fallback) instead of a degraded mode allocation so we don't uselessly enable the mode.