scs: Report SCS usage in bytes rather than number of entries

Fix the SCS debug usage check so that we report the number of bytes
used, rather than the number of entries.

Fixes: 5bbaf9d1 ("scs: Add support for stack usage debugging")
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Will Deacon <will@kernel.org>