s390/stacktrace: use common arch_stack_walk infrastructure

Use common arch_stack_walk infrastructure to avoid duplicated code and
avoid taking care of the stack storage and filtering.

Common code also uses try_get_task_stack/put_task_stack when needed which
have been missing in our code, which also solves potential problem for us.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>