CVE-2026-80682 riscv/mm: use physical alignment for vmemmap_start_pfn

In the Linux kernel, the following vulnerability has been resolved: riscv/mm: use physical alignment for vmemmapstartpfn RISC-V computes vmemmapstartpfn by rounding physrambase down to VMEMMAPADDRALIGN. That alignment must therefore be expressed in the physical-address domain. Commit 476849b0fba4...