MFC r265886, r265948
  With the new-and-improved vm_fault_copy_entry() (r265843), we can always
  avoid soft page faults when adding write access to user wired entries in
  vm_map_protect().  Previously, we only avoided the soft page fault when
  the underlying pages were copy-on-write.  In other words, we avoided the
  pages faults that might sleep on page allocation, but not the trivial
  page faults to update the physical map.

  On a fork allow read-only wired pages to be copy-on-write shared between
  the parent and child processes.  Previously, we copied these pages even
  though they are read only.  However, the reason for copying them is
  historical and no longer exists.  In recent times, vm_map_protect() has
  developed the ability to copy pages when write access is added to wired
  copy-on-write pages.  So, in this case, copy-on-write sharing of wired
  pages is not to be feared.  It is not going to lead to copy-on-write
  faults on wired memory.