[$] Revisiting the MAP_SHARED_VALIDATE hack

One of the the most commonly repeated mistakes in system-call design is a failure to check for unknown flags whereverflags are accepted. If there is ever a point where callers can get awaywith setting unknown flags, then adding new flags becomes a hazardous act.In the case of mmap(),though, developers found a clever way around this problem. A recentdiscussion has briefly called that approach into question, though, andraised the issue of what constitutes a kernel regression. No changes areforthcoming as a result, but the discussion does provide an opportunity tolook at both the specific hack and how the kernel community decides whethera change is a regression or not.