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.