powerpc/mm: Fix page table dump to work on Radix

[ Upstream commit 0d923962ab69c27cca664a2d535e90ef655110ca ]

When we're running on Book3S with the Radix MMU enabled the page table
dump currently prints the wrong addresses because it uses the wrong
start address.

Fix it to use PAGE_OFFSET rather than KERN_VIRT_START.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>