riscv: pgtable: Fixup comment for KERN_VIRT_SIZE

riscv: pgtable: Fixup comment for KERN_VIRT_SIZE

KERN_VIRT_SIZE is 1/4 of the entries of the page global directory,
not half.

Fixes: f7ae02333d13 ("riscv: Move KASAN mapping next to the kernel mapping")
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230110080419.931185-1-guoren@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>