MFC r253189:
Never remove user-wired pages from an object when doing
msync(MS_INVALIDATE). The vm_fault_copy_entry() requires that object
range which corresponds to the user-wired vm_map_entry, is always
fully populated.

Add OBJPR_NOTWIRED flag for vm_object_page_remove() to request the
preserving behaviour, use it when calling vm_object_page_remove() from
vm_object_sync().