vm_object: restore handling of shadow_count for all type of objects

instead of only OBJ_ANON objects that are backing, as it is now.
This is required for e.g. vm_meter is_object_active() detection, and
should be useful in some more cases.

Use refcount KPI for all objects, regardless of owning the object lock,
and the fact that currently OBJ_ANON cannot change for the live object.

Noted and reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33549