libnvdimm: replace mutex_is_locked() warnings with lockdep_assert_held

For warnings that should only ever trigger during development and
testing replace WARN statements with lockdep_assert_held. The lockdep
pattern is prevalent, and these paths are are well covered by libnvdimm
unit tests.

Reported-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
index 6af5e62..7cd705f 100644
--- a/drivers/nvdimm/region_devs.c
+++ b/drivers/nvdimm/region_devs.c
@@ -509,7 +509,7 @@ void nd_mapping_free_labels(struct nd_mapping *nd_mapping)
 {
 	struct nd_label_ent *label_ent, *e;
 
-	WARN_ON(!mutex_is_locked(&nd_mapping->lock));
+	lockdep_assert_held(&nd_mapping->lock);
 	list_for_each_entry_safe(label_ent, e, &nd_mapping->labels, list) {
 		list_del(&label_ent->list);
 		kfree(label_ent);