Several places (dimm_devs.c, core.c etc) include label.h but only
label.c uses NSINDEX_SIGNATURE, so move its definition to label.c
instead.
In file included from drivers/nvdimm/dimm_devs.c:23:
drivers/nvdimm/label.h:41:19: warning: 'NSINDEX_SIGNATURE' defined but
not used [-Wunused-const-variable=]
Also, some places abuse "/**" which is only reserved for the kernel-doc.