CVE-2024-57945: riscv: mm: Fix the out of bound issue of vmemmap address

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

riscv: mm: Fix the out of bound issue of vmemmap address

In sparse vmemmap model, the virtual address of vmemmap is calculated as: ((struct page )VMEMMAPSTART - (physrambase >> PAGESHIFT)). And the struct page's va can be calculated with an offset: (vmemmap + (pfn)).

However, when initializing struct pages, kernel actually starts from the first page from the same section that physrambase belongs to. If the first page's physical address is not (physrambase >> PAGESHIFT), then we get an va below VMEMMAPSTART when calculating va for it's struct page.

For example, if physrambase starts from 0x82000000 with pfn 0x82000, the first page in the same section is actually pfn 0x80000. During initunavailablerange(), we will initialize struct page for pfn 0x80000 with virtual address ((struct page )VMEMMAPSTART - 0x2000), which is below VMEMMAPSTART as well as PCIIOEND.

This commit fixes this bug by introducing a new variable 'vmemmapstartpfn' which is aligned with memory section size and using it to calculate vmemmap address instead of physrambase.