Fix crash with FLAT_MEMORY and ARCH_PFN_OFFSET != 0

When using FLAT_MEMORY and ARCH_PFN_OFFSET is not 0, the kernel crashes in
memmap_init_zone(). This bug got introduced by commit
c713216deebd95d2b0ab38fef8bb2361c0180c2d

Signed-off-by: Thomas Bogendoerfer <[e-mail]>
Acked-by: Mel Gorman <[e-mail]>
Cc: Bob Picco <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Andy Whitcroft <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: "Keith Mannthey" <[e-mail]>
Cc: "Luck, Tony" <[e-mail]>
Cc: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Yasunori Goto <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/page_alloc.c

···
 		mem_map = NODE_DATA(0)->node_mem_map;
 #ifdef CONFIG_ARCH_POPULATES_NODE_MAP
 		if (page_to_pfn(mem_map) != pgdat->node_start_pfn)
-			mem_map -= pgdat->node_start_pfn;
+			mem_map -= (pgdat->node_start_pfn - ARCH_PFN_OFFSET);
 #endif /* CONFIG_ARCH_POPULATES_NODE_MAP */
 	}
 #endif