csky: Fixup pfn_valid error with wrong max_mapnr

The max_mapnr is the number of PFNs, not absolute PFN offset.
Using set_max_mapnr API instead of setting the value directly.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
diff --git a/arch/csky/mm/init.c b/arch/csky/mm/init.c
index bc05a3b..81e4e5e 100644
--- a/arch/csky/mm/init.c
+++ b/arch/csky/mm/init.c
@@ -86,9 +86,9 @@
 #ifdef CONFIG_HIGHMEM
 	unsigned long tmp;
 
-	max_mapnr = highend_pfn;
+	set_max_mapnr(highend_pfn - ARCH_PFN_OFFSET);
 #else
-	max_mapnr = max_low_pfn;
+	set_max_mapnr(max_low_pfn - ARCH_PFN_OFFSET);
 #endif
 	high_memory = (void *) __va(max_low_pfn << PAGE_SHIFT);