For vm_map, #defining DIAGNOSTIC to turn on full assertion-based
consistency checking slows performance dramatically. This change
reduces the number of assertions checked by completely walking the
vm_map tree only when the write-lock is released, and only then if the
number of modifications to the tree since the last walk exceeds the
number of tree nodes.

Reviewed by: alc, kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D22163