Fix Coverity issue in the NVDIMM driver

Summary:
Coverity reports a potential memory leak in the nvdimm
driver. Examination shows it's real; fix it.

Sponsored by: Dell Technologies
MFC after: 1w

Test Plan: Changes in use at $WORK

Reviewers: robert.herndon_dell.com, vangyzen, bret_ketchum_dell.com

Subscribers: imp, badger

Differential Revision: https://reviews.freebsd.org/D38676