riscv: Extend sv39 linear mapping max size to 128G

This harmonizes all virtual addressing modes which can now all map
(PGDIR_SIZE * PTRS_PER_PGD) / 4 of physical memory.

The RISCV implementation of KASAN requires that the boundary between
shallow mappings are aligned on an 8G boundary. In this case we need
VMALLOC_START to be 8G aligned. So although we only need to move the
start of the linear mapping down by 4GiB to allow 128GiB to be mapped,
we actually move it down by 8GiB (creating a 4GiB hole between the
linear mapping and KASAN shadow space) to maintain the alignment
requirement.

Signed-off-by: Stuart Menefy <stuart.menefy@codasip.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240630110550.1731929-1-stuart.menefy@codasip.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>