mm/page_alloc: don't call pfn_to_page() on possibly non-existent PFN in split_large_buddy()

In the Linux kernel, the following vulnerability has been resolved:

mm/page_alloc: don't call pfn_to_page() on possibly non-existent PFN in split_large_buddy()

In split_large_buddy(), we might call pfn_to_page() on a PFN that might
not exist. In corner cases, such as when freeing the highest pageblock in
the last memory section, this could result with CONFIG_SPARSEMEM &&
!CONFIG_SPARSEMEM_EXTREME in __pfn_to_section() returning NULL and and
__section_mem_map_addr() dereferencing that NULL pointer.

Let's fix it, and avoid doing a pfn_to_page() call for the first
iteration, where we already have the page.

So far this was found by code inspection, but let's just CC stable as the
fix is easy.