riscv: mm: Fix out-of-bounds page-table walk during memory hot-remove

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

riscv: mm: Fix out-of-bounds page-table walk during memory hot-remove

remove_pud_mapping() and remove_p4d_mapping() obtain a child table base
with pud_offset(p4dp, 0) and p4d_offset(pgd, 0), then add the index for
addr.

RISC-V folds page-table levels at runtime. When a level is folded, its
offset helper returns the parent entry itself, but the index can still be
nonzero. Adding it walks past the parent table. Sv48 folds P4D, while Sv39
folds both P4D and PUD, so memory hot-remove can descend into unrelated
memory and pass an invalid page to __free_pages(). This can trigger:

kernel BUG at include/linux/mm.h:1810!
VM_BUG_ON_PAGE(page_ref_count(page) == 0)
arch_remove_memory+0x1e/0x5c
try_remove_memory+0x15e/0x200
remove_memory+0x24/0x3c

Only add the index when the corresponding page-table level is enabled,
matching p4d_offset() and pud_offset().