x86/signal: Fix the value returned by strict_sas_size()

Functions used with __setup() return 1 when the argument has been
successfully parsed.

Reverse the returned value so that 1 is returned when kstrtobool() is
successful (i.e. returns 0).

My understanding of these __setup() functions is that returning 1 or 0
does not change much anyway - so this is more of a cleanup than a
functional fix.

I spot it and found it spurious while looking at something else.
Even if the output is not perfect, you'll get the idea with:

   $ git grep -B2 -A10 retu.*kstrtobool | grep __setup -B10

Fixes: 3aac3ebe

 ("x86/signal: Implement sigaltstack size validation")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/73882d43ebe420c9d8fb82d0560021722b243000.1673717552.git.christophe.jaillet@wanadoo.fr