riscv/mm: Introduce a die_kernel_fault() helper function

Like arm64, this patch adds a die_kernel_fault() helper
to ensure the same semantics for the different kernel faults.

Signed-off-by: Eric Lin <tesheng@andestech.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>