kasan: don't round_up too much

For tag-based mode kasan_poison_memory() already rounds up the size. Do
the same for software modes and remove round_up() from common code.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Link: https://linux-review.googlesource.com/id/Ib397128fac6eba874008662b4964d65352db4aa4