Move the definition of _vm_map_assert_consistent so that it can use
vm_map_free_{left,right} rather than re-implementing them. Use the
VM_MAP_FOREACH macro where applicable. Fix some indentation.

Suggested by: kib (in a comment on D21964)
Tested by: pho (as part of D21964)
Differential Revision: https://reviews.freebsd.org/D22011