mm: add stubs for PFNMAP memory failure registration functions

Add stubs to address CONFIG_MEMORY_FAILURE disabled.

Suggested-by: Alex Williamson <alex@shazbot.org>
Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
Link: https://lore.kernel.org/r/20260115202849.2921-2-ankita@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

Author: ankita-nv

include/linux/memory-failure.h

@@ -4,16 +4,25 @@
 
 #include <linux/interval_tree.h>
 
-struct pfn_address_space;
-
 struct pfn_address_space {
 	struct interval_tree_node node;
 	struct address_space *mapping;
 	int (*pfn_to_vma_pgoff)(struct vm_area_struct *vma,
 				unsigned long pfn, pgoff_t *pgoff);
 };
 
+#ifdef CONFIG_MEMORY_FAILURE
 int register_pfn_address_space(struct pfn_address_space *pfn_space);
 void unregister_pfn_address_space(struct pfn_address_space *pfn_space);
+#else
+static inline int register_pfn_address_space(struct pfn_address_space *pfn_space)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline void unregister_pfn_address_space(struct pfn_address_space *pfn_space)
+{
+}
+#endif /* CONFIG_MEMORY_FAILURE */
 
 #endif /* _LINUX_MEMORY_FAILURE_H */