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 pfn_section_valid()

Commit 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing

memory_section->usage") changed pfn_section_valid() to add a READ_ONCE()

call around "ms->usage" to fix a race with section_deactivate() where

ms->usage can be cleared. The READ_ONCE() call, by itself, is not enough

to prevent NULL pointer dereference. We need to check its value before

dereferencing it.