Sentinel -1 collision: fuwordadd32 fault indistinguishable from mutex value -1

Summary

fuwordadd32 returns -1 on fault(support.s:564). Value -1 is valid mutex token. Loop while(waddr==-1&&value!=-1) and if(value==-1&&waddr==-1)->EINVAL cant tell fault from value. COW/pageout race on token=-1 -> EINVAL instead of sleep.