[PATCH] libnvdimm/e820: Fix build error without MEMORY_HOTPLUG

[YueHaibing]

If MEMORY_HOTPLUG is n, build fails:

drivers/nvdimm/e820.c: In function ‘e820_register_one’:
drivers/nvdimm/e820.c:24:12: error: implicit declaration of function ‘phys_to_target_node’; did you mean ‘set_page_node’? [-Werror=implicit-function-declaration]
  int nid = phys_to_target_node(res->start);
            ^~~~~~~~~~~~~~~~~~~
            set_page_node

Fixes: 7b27a8622f80 ("libnvdimm/e820: Retrieve and populate correct 'target_node' info")
Signed-off-by: YueHaibing <yuehaibing@xxxxxxxxxx>
---
 drivers/nvdimm/e820.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/nvdimm/e820.c b/drivers/nvdimm/e820.c
index 4cd18be9d0e9..c741f029f215 100644
--- a/drivers/nvdimm/e820.c
+++ b/drivers/nvdimm/e820.c
@@ -17,6 +17,13 @@ static int e820_pmem_remove(struct platform_device *pdev)
 	return 0;
 }
 
+#ifndef CONFIG_MEMORY_HOTPLUG
+static inline int phys_to_target_node(u64 start)
+{
+	return 0;
+}
+#endif
+
 static int e820_register_one(struct resource *res, void *data)
 {
 	struct nd_region_desc ndr_desc;
-- 
2.17.1