nvdimm(4): Only expose namespaces for accessible data SPAs

Apply the same user accessible filter to namespaces as is applied
to full-SPA devices. Also, explicitly filter out control region
SPAs which don't expose the nvdimm data area.

Reviewed by: cem
Approved by: scottl (mentor)
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D21987