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>