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>

drivers/nvdimm/bus.c

@@ -642,7 +642,7 @@ static struct attribute *nd_device_attributes[] = {
 	NULL,
 };
 
-/**
+/*
  * nd_device_attribute_group - generic attributes for all devices on an nd bus
  */
 struct attribute_group nd_device_attribute_group = {
@@ -671,7 +671,7 @@ static umode_t nd_numa_attr_visible(struct kobject *kobj, struct attribute *a,
 	return a->mode;
 }
 
-/**
+/*
  * nd_numa_attribute_group - NUMA attributes for all devices on an nd bus
  */
 struct attribute_group nd_numa_attribute_group = {

drivers/nvdimm/label.c

@@ -25,6 +25,8 @@ static guid_t nvdimm_btt2_guid;
 static guid_t nvdimm_pfn_guid;
 static guid_t nvdimm_dax_guid;
 
+static const char NSINDEX_SIGNATURE[] = "NAMESPACE_INDEX\0";
+
 static u32 best_seq(u32 a, u32 b)
 {
 	a &= NSINDEX_SEQ_MASK;

drivers/nvdimm/label.h

@@ -38,8 +38,6 @@ enum {
 	ND_NSINDEX_INIT = 0x1,
 };
 
-static const char NSINDEX_SIGNATURE[] = "NAMESPACE_INDEX\0";
-
 /**
  * struct nd_namespace_index - label set superblock
  * @sig: NAMESPACE_INDEX\0