target-s390x: Only access allocated storage keys

We allocate ram_size / PAGE_SIZE storage keys, so we need to make sure that
we only access that many. Unfortunately the code can overrun this array by
one, potentially overwriting unrelated memory.

Fix it by limiting storage keys to their scope.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
