riscv: Improve kasan definitions

There is no functional change here, only improvement in code readability
by adding comments to explain where the kasan constants come from and by
replacing hardcoded numerical constant by the corresponding define.

Note that the comments come from arm64.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>