Merge r349526 from amd64. When we protect an L3 entry, we only call
vm_page_dirty() when, in fact, we are write protecting the page and the L3
entry has PTE_D set. However, pmap_protect() was always calling
vm_page_dirty() when an L2 entry has PTE_D set. Handle L2 entries the
same as L3 entries so that we won't perform unnecessary calls to
vm_page_dirty().

Simplify the loop calling vm_page_dirty() on L2 entries.