kasan: remove __kasan_unpoison_stack

There's no need for __kasan_unpoison_stack() helper, as it's only
currently used in a single place. Removing it also removes undeed
arithmetic.

No functional changes.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Link: https://linux-review.googlesource.com/id/Ie5ba549d445292fe629b4a96735e4034957bcc50