ARM: 8012/1: kdump: Avoid overflow when converting pfn to physaddr

When we configure CONFIG_ARM_LPAE=y, pfn << PAGE_SHIFT will
overflow if pfn >= 0x100000 in copy_oldmem_page.
So use __pfn_to_phys for converting.

Signed-off-by: Liu Hua <sdu.liu@huawei.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>