riscv: Improve arch_get_mmap_end() macro

riscv: Improve arch_get_mmap_end() macro

This macro caused me some confusion, which took some reviewer's time to
make it clear, so I propose adding a short comment in code to avoid
confusion in the future.

Also, added some improvements to the macro, such as removing the
assumption of VA_USER_SV57 being the largest address space.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20240103160024.70305-3-leobras@redhat.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>