Improve pmap protect

[franciscozdo]

Optimizing pmap_protect to traverse pmap structure effectively instead of looking up every pte starting from root.

The pmap_protect_walk function is called recursively at each pmap level. On the last level it modifies the protection of pte.

Below I provide some statistics about function calls created using KFT, to show that this change really matters.

Before

     function: count avg time
 vm_map_clone:   146   158477
vm_page_fault:  3910     7901
 pmap_protect:  1041    19849

After

     function: count avg time
 vm_map_clone:   146    64040
vm_page_fault:  3668     7554
 pmap_protect:  1041     6674

[cahirwpz]

A few nit-picks, otherwise looks good.

[cahirwpz]

LGTM