raid6: riscv: Allow code to be compiled in userspace

raid6: riscv: Allow code to be compiled in userspace

To support userspace raid6test, this patch adds __KERNEL__ ifdef for kernel
header inclusions also userspace wrapper definitions to allow code to be
compiled in userspace.

This patch also drops the NSIZE macro, instead of using the vector length,
which can work for both kernel and user space.

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Chunyan Zhang <zhangchunyan@iscas.ac.cn>
Link: https://patch.msgid.link/20250718072711.3865118-5-zhangchunyan@iscas.ac.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>