powerpc: start loop at section start of start in vmemmap_populated()

vmemmap_populated() checks whether the [start, start + page_size) has valid
pfn numbers, to know whether a vmemmap mapping has been created that includes
this range.

Some range before end might not be checked by this loop:
  sec11start......start11..sec11end/sec12start..end....start12..sec12end
as the above, for start11(section 11), it checks [sec11start, sec11end), and
loop ends as the next start(start12) is bigger than end. However,
[sec11end/sec12start, end) is not checked here.

So before the loop, adjust the start to be the start of the section, so we don't miss ranges like the above.

After we adjust start to be the start of the section, it also means it's
aligned with vmemmap as of the sizeof struct page, so we could use
page_to_pfn directly in the loop.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Acked-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>