Change *_pager_allocate() to take off_t instead of vm_ooffset_t. The

Change *_pager_allocate() to take off_t instead of vm_ooffset_t.  The
actual underlying type (a 64 bit signed integer) is the same.   Recent and
upcoming work is standardizing on off_t.

Move object->un_pager.vnp.vnp_size to vnode->v_filesize.  As before, the
field is still only valid when a VM object is associated with the vnode.