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.

Approved by: markj
Tested by: pho (as part of a larger patch)
Differential Revision: https://reviews.freebsd.org/D22347