The map-entry clipping functions modify start and end entries of an
entry in the vm_map, making invariants related to the max_free entry
field invalid. Move the clipping work into vm_map_entry_link, so that
linking is okay when the new entry clips a current entry, and the
vm_map doesn't have to be briefly corrupted. Change assertions and
conditions in SPLAY_{LEFT,RIGHT}_STEP since the max_free invariants
can now be trusted in all cases.

Tested by: pho
Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D22897