[PATCH x86,mm] fix pgt_buf_end when memory hotplug

[Lai Jiangshan <laijs@cn.fujitsu.com>]

When I test memory hotplug, such message is always outputted when hot-adding
a memory block:

initial kernel pagetable allocation wasted ffffffffffffffff pages

This message comes from init_memory_mapping(), it shows "pgt_buf_end" is
incorrect here, and I found it was caused by alloc_low_page().

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index bbaaa00..e138e0a 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -317,7 +317,7 @@ void __init cleanup_highmap(void)
 
 static __ref void *alloc_low_page(unsigned long *phys)
 {
-	unsigned long pfn = pgt_buf_end++;
+	unsigned long pfn;
 	void *adr;
 
 	if (after_bootmem) {
@@ -327,6 +327,7 @@ static __ref void *alloc_low_page(unsigned long *phys)
 		return adr;
 	}
 
+	pfn = pgt_buf_end++;
 	if (pfn >= pgt_buf_top)
 		panic("alloc_low_page: ran out of memory");