nvdimm(4): Fix various problems when the using the second label index block

struct nvdimm_label_index is dynamically sized, with the free
bitfield expanding to hold slot_cnt entries. Fix a few places
where we were treating the struct as though it had a fixed sized.

Reviewed by: cem
Approved by: scottl (mentor)
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D22253