hide map entry next and prev fields

[dougm]

Summary

Define wrapper functions vm_map_entry_{succ,pred} to act as wrappers around entry->{next,prev} when those are used for ordered list traversal, and use those wrapper functions everywhere. Where the next field is used for maintaining a stack of deferred operations, #define defer_next to make that different usage clearer, and then use the 'right' pointer instead of 'next' for that purpose.

[markj]

Thanks. If you plan to commit this patch on its own, I'd suggest adding a comment above defer_next, even though that definition would be deleted shortly after.