powerpc: Fix /dev/oldmem interface for kdump

A change to __ioremap() broke reading /dev/oldmem because we're no
longer able to ioremap pfn 0 (d177c207, "[PATCH] powerpc: IOMMU: don't
ioremap null addresses").

We actually don't need to ioremap for anything that's part of the linear
mapping, so just read it directly.

Also make sure we're only reading one page or less at a time.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>