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.

Reviewed by:	kib
MFC after:	1 week
Sponsored by:	EMC / Isilon Storage Division