kernel: mm: prevent derefencing NULL ptr in pfn_section_valid()

In the Linux kernel, the following vulnerability has been resolved: mm: prevent derefencing NULL ptr in pfnsectionvalid Commit 5ec8e8ea8b77 "mm/sparsemem: fix race in accessing memorysection-usage" changed pfnsectionvalid to add a READONCE call around "ms-usage" to fix a race with sectiondeactiva...