memmap should be the address to page struct instead of address to pfn.

As mentioned by David, if system memory and devmem sit within a section,
the mismatch address would lead kdump to dump unexpected memory.

Since sub-section only works for SPARSEMEM_VMEMMAP, pfn_to_page() is valid
to get the page struct address at this point.

Fixes: ba72b4c8 ("mm/sparsemem: support sub-section hotplug")
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Baoquan He <bhe@redhat.com>
Link: http://lkml.kernel.org/r/20200210005048.10437-1-richardw.yang@linux.intel.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>