libnvdimm: Fix compilation warnings with W=1

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.

drivers/nvdimm/bus.c:648: warning: cannot understand function prototype:
'struct attribute_group nd_device_attribute_group = '
drivers/nvdimm/bus.c:677: warning: cannot understand function prototype:
'struct attribute_group nd_numa_attribute_group = '

Those are just some member assignments for the "struct attribute_group"
instances and it can't be expressed in the kernel-doc.

Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>