ARM: 5747/1: Fix the start_pg value in free_memmap()

If sparsemem is enabled, the start_pfn passed to the free_memmap()
function corresponds to an area of memory not known to the kernel and
pfn_to_page returns a wrong value. The (start_pfn - 1), however, is
known to the kernel.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/init.c

@@ -483,7 +483,7 @@ free_memmap(int node, unsigned long start_pfn, unsigned long end_pfn)
 	/*
 	 * Convert start_pfn/end_pfn to a struct page pointer.
 	 */
-	start_pg = pfn_to_page(start_pfn);
+	start_pg = pfn_to_page(start_pfn - 1) + 1;
 	end_pg = pfn_to_page(end_pfn);
 
 	/*