Remove the next and prev fields from vm_map_entry, to save a bit of
space. Where the vm_map tree now has null pointers, store pointers to
next and previous entries in right and left fields, making the binary
tree threaded. Have the predecessor and successor functions compute
what the prev and next fields previously stored.

Reviewed by: markj, kib (previous version)
Tested by: pho (previous version)
Differential Revision: https://reviews.freebsd.org/D21964