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 vmemmap_start_pfn

RISC-V computes vmemmap_start_pfn by rounding phys_ram_base down to
VMEMMAP_ADDR_ALIGN. That alignment must therefore be expressed in the
physical-address domain.

Commit 476849b0fba4 ("riscv/mm: align vmemmap to maximal folio size")
attempted to account for the maximal folio alignment by feeding
MAX_FOLIO_VMEMMAP_ALIGN directly into VMEMMAP_ADDR_ALIGN. However,
MAX_FOLIO_VMEMMAP_ALIGN is measured in bytes of struct page storage,
whereas VMEMMAP_ADDR_ALIGN is used to align a physical address.

The mask-based compound_info encoding requires pfn_to_page(0) to be
naturally aligned to MAX_FOLIO_VMEMMAP_ALIGN. Commit 9f94db4c7eaa
("mm/sparse: check memmap alignment for compound_info_has_mask()") added a
check for that requirement and exposed the unit mismatch on systems such
as QEMU virt, where the DRAM base is not aligned to MAX_FOLIO_NR_PAGES *
PAGE_SIZE.

Here is the log:
[ 0.000000][ C0] ------------[ cut here ]------------
[ 0.000000][ C0] WARNING: mm/sparse.c:365 at sparse_init+0x58a/0x6fe, CPU#0: swapper/0
[ 0.000000][ C0] Modules linked in:
[ 0.000000][ C0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.2.0-rc3-g1d8304bdd65f #2 PREEMPT
[ 0.000000][ C0] Hardware name: riscv-virtio,qemu (DT)
[ 0.000000][ C0] epc : sparse_init+0x58a/0x6fe
[ 0.000000][ C0] ra : sparse_init+0x58a/0x6fe
[ 0.000000][ C0] epc : ffffffff86851c88 ra : ffffffff86851c88 sp : ffffffff88807a30
[ 0.000000][ C0] gp : ffffffff8a3bf240 tp : ffffffff88842080 t0 : ff600000ffab6000
[ 0.000000][ C0] t1 : 000000017fab6000 t2 : 65203a6573726363 s0 : ffffffff88807bc0
[ 0.000000][ C0] s1 : 000000000e000000 a0 : 0000000000000007 a1 : 0000000000000000
[ 0.000000][ C0] a2 : 0000000000000002 a3 : ffffffff86851c88 a4 : 0000000000000000
[ 0.000000][ C0] a5 : ffffffff88843080 a6 : 0000000000000003 a7 : 0000000000000000
[ 0.000000][ C0] s2 : ff60000000000000 s3 : 0040000000000000 s4 : 0004000000000000
[ 0.000000][ C0] s5 : ffffffff8a4d92e0 s6 : ff600000ffab55e0 s7 : ffffffff88384d00
[ 0.000000][ C0] s8 : 0000000000000003 s9 : ffffffff88384cc1 s10: ffffffff88384cc0
[ 0.000000][ C0] s11: ffffffff8a4daae0 t3 : ffffffff915e8b20 t4 : ffffffff915e8b20
[ 0.000000][ C0] t5 : ffffffff915e8b20 t6 : ffffffff915e8bc8 ssp : 0000000000000000
[ 0.000000][ C0] status: 0000000200000100 badaddr: ffffffff86851c88 cause: 0000000000000003
[ 0.000000][ C0] [<ffffffff86851c88>] sparse_init+0x58a/0x6fe
[ 0.000000][ C0] [<ffffffff8683d396>] mm_core_init_early+0x116/0x1e30
[ 0.000000][ C0] [<ffffffff86801edc>] start_kernel+0xd2/0x848

Convert MAX_FOLIO_VMEMMAP_ALIGN to the equivalent physical alignment
before using it in VMEMMAP_ADDR_ALIGN. This keeps the existing
round_down() logic while making the resulting vmemmap base satisfy the
mask-alignment requirement.