riscv: s64ilp32: Adjust TASK_SIZE for s64ilp32 kernel

The RV64ILP32 32-bit Linux kernel uses the same userspace address range
as the 64-bit Linux compat mode, about 2GB. They have no difference from
the hardware view, and all are running ILP32 on a 64-bit ISA. But the
standard 32ilp32 Linux has a slightly bigger userspace address space,
about 2.4GB.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>