locking/atomic: scripts: remove leftover "${mult}"

We removed cmpxchg_double() and variants in commit:

  b4cf83b2d1da40b2 ("arch: Remove cmpxchg_double")

Which removed the need for "${mult}" in the instrumentation logic.
Unfortunately we missed an instance of "${mult}".

There is no change to the generated header.
There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230605070124.3741859-16-mark.rutland@arm.com