memremap: provide stubs for vmem_altmap_offset and vmem_altmap_free

Currently all calls to those functions are eliminated by the compiler when
CONFIG_ZONE_DEVICE is not set, but this soon won't be the case.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

include/linux/memremap.h

@@ -26,9 +26,6 @@ struct vmem_altmap {
 	unsigned long alloc;
 };
 
-unsigned long vmem_altmap_offset(struct vmem_altmap *altmap);
-void vmem_altmap_free(struct vmem_altmap *altmap, unsigned long nr_pfns);
-
 #ifdef CONFIG_ZONE_DEVICE
 struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start);
 #else
@@ -138,6 +135,9 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
 		struct percpu_ref *ref, struct vmem_altmap *altmap);
 struct dev_pagemap *find_dev_pagemap(resource_size_t phys);
 
+unsigned long vmem_altmap_offset(struct vmem_altmap *altmap);
+void vmem_altmap_free(struct vmem_altmap *altmap, unsigned long nr_pfns);
+
 static inline bool is_zone_device_page(const struct page *page);
 #else
 static inline void *devm_memremap_pages(struct device *dev,
@@ -157,7 +157,17 @@ static inline struct dev_pagemap *find_dev_pagemap(resource_size_t phys)
 {
 	return NULL;
 }
-#endif
+
+static inline unsigned long vmem_altmap_offset(struct vmem_altmap *altmap)
+{
+	return 0;
+}
+
+static inline void vmem_altmap_free(struct vmem_altmap *altmap,
+		unsigned long nr_pfns)
+{
+}
+#endif /* CONFIG_ZONE_DEVICE */
 
 #if defined(CONFIG_DEVICE_PRIVATE) || defined(CONFIG_DEVICE_PUBLIC)
 static inline bool is_device_private_page(const struct page *page)