s390/unwind: unify task is current checks

Avoid mixture of task == NULL and task == current meaning the same
thing and simply always initialize task with current in unwind_start.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
3 files changed