s390/process: Fix kernel thread function pointer type
In case of a kernel thread __ret_from_fork() calls the specified function
indirectly. Fix the kernel thread function pointer, since kernel threads
return an int instead of void.
Fixes: 56e62a737028 ("s390: convert to generic entry") Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com> Signed-off-by: Heiko Carstens <hca@linux.ibm.com> Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>